Walkthrough: Displaying Matching Braces

You can implement language-based features such as brace matching by defining the braces you want to match, and then adding a text marker tag to the matching braces when the caret is on one of the braces. You can define braces in the context of a language, or you can define your own file name extension and content type and apply the tags to just that type, or you can apply the tags to an existing content type (such as "text"). The following walkthrough shows how to apply brace matching tags to the "text" content type.

Prerequisites

Starting in Visual Studio 2015, you do not install the Visual Studio SDK from the download center. It is included as an optional feature in Visual Studio setup. You can also install the VS SDK later on. For more information, see Installing the Visual Studio SDK.

Creating a Managed Extensibility Framework (MEF) Project

To create a MEF project

1. Create an Editor Classifier project. Name the solution BraceMatchingTest.

2. Add an Editor Classifier item template to the project. For more information, see Creating an Extension with an Editor Item Template.

3. Delete the existing class files.

Implementing a Brace Matching Tagger

To get a brace highlighting effect that resembles the one that is used in Visual Studio, you can implement a tagger of type TextMarkerTag. The following code shows how to define the tagger for brace pairs at any level of nesting. In this example, the brace pairs []. [], and {} are defined in the tagger constructor, but in a full language implementation the relevant brace pairs would be defined in the language specification.

To implement a brace matching tagger

1. Add a class file and name it BraceMatching.

2. Import the following namespaces.

   using System;
   using System.Linq;
   using System.Collections.Generic;
   using System.ComponentModel.Composition;
   using Microsoft.VisualStudio.Text;
   using Microsoft.VisualStudio.Text.Editor;
   using Microsoft.VisualStudio.Text.Tagging;
   using Microsoft.VisualStudio.Utilities;
   

   Imports System.ComponentModel.Composition
   Imports Microsoft.VisualStudio.Text
   Imports Microsoft.VisualStudio.Text.Editor
   Imports Microsoft.VisualStudio.Text.Tagging
   Imports Microsoft.VisualStudio.Utilities
   

3. Define a class BraceMatchingTagger that inherits from ITagger<T> of type TextMarkerTag.

   internal class BraceMatchingTagger : ITagger<TextMarkerTag>
   

   Friend Class BraceMatchingTagger
       Implements ITagger(Of TextMarkerTag)
   

4. Add properties for the text view, the source buffer, and the current snapshot point, and also a set of brace pairs.

   ITextView View { get; set; }
   ITextBuffer SourceBuffer { get; set; }
   SnapshotPoint? CurrentChar { get; set; }
   private Dictionary<char, char> m_braceList;
   

   Private _View As ITextView
   Private Property View() As ITextView
       Get
           Return _View
       End Get
       Set(ByVal value As ITextView)
           _View = value
       End Set
   End Property
   Private _SourceBuffer As ITextBuffer
   Private Property SourceBuffer() As ITextBuffer
       Get
           Return _SourceBuffer
       End Get
       Set(ByVal value As ITextBuffer)
           _SourceBuffer = value
       End Set
   End Property
   Private _CurrentChar As System.Nullable(Of SnapshotPoint)
   Private Property CurrentChar() As System.Nullable(Of SnapshotPoint)
       Get
           Return _CurrentChar
       End Get
       Set(ByVal value As System.Nullable(Of SnapshotPoint))
           _CurrentChar = value
       End Set
   End Property
   Private m_braceList As Dictionary(Of Char, Char)
   

5. In the tagger constructor, set the properties and subscribe to the view change events PositionChanged and LayoutChanged. In this example, for illustrative purposes, the matching pairs are also defined in the constructor.

   internal BraceMatchingTagger(ITextView view, ITextBuffer sourceBuffer)
   {
       //here the keys are the open braces, and the values are the close braces
       m_braceList = new Dictionary<char, char>();
       m_braceList.Add('{', '}');
       m_braceList.Add('[', ']');
       m_braceList.Add('(', ')');
       this.View = view;
       this.SourceBuffer = sourceBuffer;
       this.CurrentChar = null;
   
       this.View.Caret.PositionChanged += CaretPositionChanged;
       this.View.LayoutChanged += ViewLayoutChanged;
   }
   

   Friend Sub New(ByVal view As ITextView, ByVal sourceBuffer As ITextBuffer)
       'here the keys are the open braces, and the values are the close braces
       m_braceList = New Dictionary(Of Char, Char)()
       m_braceList.Add("{"c, "}"c)
       m_braceList.Add("["c, "]"c)
       m_braceList.Add("("c, ")"c)
       Me.View = view
       Me.SourceBuffer = sourceBuffer
       Me.CurrentChar = Nothing
   
       AddHandler Me.View.Caret.PositionChanged, AddressOf Me.CaretPositionChanged
       AddHandler Me.View.LayoutChanged, AddressOf Me.ViewLayoutChanged
   End Sub
   

6. As part of the ITagger<T> implementation, declare a TagsChanged event.

   public event EventHandler<SnapshotSpanEventArgs> TagsChanged;
   

   Public Event TagsChanged As EventHandler(Of SnapshotSpanEventArgs) _
       Implements ITagger(Of TextMarkerTag).TagsChanged
   

7. The event handlers update the current caret position of the CurrentChar property and raise the TagsChanged event.

   void ViewLayoutChanged(object sender, TextViewLayoutChangedEventArgs e)
   {
       if (e.NewSnapshot != e.OldSnapshot) //make sure that there has really been a change
       {
           UpdateAtCaretPosition(View.Caret.Position);
       }
   }
   
   void CaretPositionChanged(object sender, CaretPositionChangedEventArgs e)
   {
       UpdateAtCaretPosition(e.NewPosition);
   }
   void UpdateAtCaretPosition(CaretPosition caretPosition)
   {
       CurrentChar = caretPosition.Point.GetPoint(SourceBuffer, caretPosition.Affinity);
   
       if (!CurrentChar.HasValue)
           return;
   
       var tempEvent = TagsChanged;
       if (tempEvent != null)
           tempEvent(this, new SnapshotSpanEventArgs(new SnapshotSpan(SourceBuffer.CurrentSnapshot, 0,
               SourceBuffer.CurrentSnapshot.Length)));
   }
   

   Private Sub ViewLayoutChanged(ByVal sender As Object, ByVal e As TextViewLayoutChangedEventArgs)
       If e.NewSnapshot IsNot e.OldSnapshot Then
           'make sure that there has really been a change
           UpdateAtCaretPosition(View.Caret.Position)
       End If
   End Sub
   
   Private Sub CaretPositionChanged(ByVal sender As Object, ByVal e As CaretPositionChangedEventArgs)
       UpdateAtCaretPosition(e.NewPosition)
   End Sub
   
   Private Sub UpdateAtCaretPosition(ByVal caretPosition As CaretPosition)
       CurrentChar = caretPosition.Point.GetPoint(SourceBuffer, caretPosition.Affinity)
   
       If Not CurrentChar.HasValue Then
           Exit Sub
       End If
   
       RaiseEvent TagsChanged(Me, New SnapshotSpanEventArgs(New SnapshotSpan(SourceBuffer.CurrentSnapshot, 0, SourceBuffer.CurrentSnapshot.Length)))
   End Sub
   

8. Implement the GetTags method to match braces either when the current character is an open brace or when the previous character is a close brace, as in Visual Studio. When the match is found, this method instantiates two tags, one for the open brace and one for the close brace.

   public IEnumerable<ITagSpan<TextMarkerTag>> GetTags(NormalizedSnapshotSpanCollection spans)
   {
       if (spans.Count == 0)   //there is no content in the buffer
           yield break;
   
       //don't do anything if the current SnapshotPoint is not initialized or at the end of the buffer
       if (!CurrentChar.HasValue || CurrentChar.Value.Position >= CurrentChar.Value.Snapshot.Length)
           yield break;
   
       //hold on to a snapshot of the current character
       SnapshotPoint currentChar = CurrentChar.Value;
   
       //if the requested snapshot isn't the same as the one the brace is on, translate our spans to the expected snapshot
       if (spans[0].Snapshot != currentChar.Snapshot)
       {
           currentChar = currentChar.TranslateTo(spans[0].Snapshot, PointTrackingMode.Positive);
       }
   
       //get the current char and the previous char
       char currentText = currentChar.GetChar();
       SnapshotPoint lastChar = currentChar == 0 ? currentChar : currentChar - 1; //if currentChar is 0 (beginning of buffer), don't move it back
       char lastText = lastChar.GetChar();
       SnapshotSpan pairSpan = new SnapshotSpan();
   
       if (m_braceList.ContainsKey(currentText))   //the key is the open brace
       {
           char closeChar;
           m_braceList.TryGetValue(currentText, out closeChar);
           if (BraceMatchingTagger.FindMatchingCloseChar(currentChar, currentText, closeChar, View.TextViewLines.Count, out pairSpan) == true)
           {
               yield return new TagSpan<TextMarkerTag>(new SnapshotSpan(currentChar, 1), new TextMarkerTag("blue"));
               yield return new TagSpan<TextMarkerTag>(pairSpan, new TextMarkerTag("blue"));
           }
       }
       else if (m_braceList.ContainsValue(lastText))    //the value is the close brace, which is the *previous* character 
       {
           var open = from n in m_braceList
                      where n.Value.Equals(lastText)
                      select n.Key;
           if (BraceMatchingTagger.FindMatchingOpenChar(lastChar, (char)open.ElementAt<char>(0), lastText, View.TextViewLines.Count, out pairSpan) == true)
           {
               yield return new TagSpan<TextMarkerTag>(new SnapshotSpan(lastChar, 1), new TextMarkerTag("blue"));
               yield return new TagSpan<TextMarkerTag>(pairSpan, new TextMarkerTag("blue"));
           }
       }
   }
   

   Public Function GetTags(ByVal spans As NormalizedSnapshotSpanCollection) As IEnumerable(Of ITagSpan(Of TextMarkerTag)) Implements ITagger(Of Microsoft.VisualStudio.Text.Tagging.TextMarkerTag).GetTags
       If spans.Count = 0 Then
           'there is no content in the buffer
           Exit Function
       End If
   
       'don't do anything if the current SnapshotPoint is not initialized or at the end of the buffer
       If Not CurrentChar.HasValue OrElse CurrentChar.Value.Position >= CurrentChar.Value.Snapshot.Length Then
           Exit Function
       End If
   
       'hold on to a snapshot of the current character
       Dim currentChar__1 As SnapshotPoint = CurrentChar.Value
   
       'if the requested snapshot isn't the same as the one the brace is on, translate our spans to the expected snapshot
       If spans(0).Snapshot IsNot currentChar__1.Snapshot Then
           currentChar__1 = currentChar__1.TranslateTo(spans(0).Snapshot, PointTrackingMode.Positive)
       End If
   
       'get the current char and the previous char
       Dim currentText As Char = currentChar__1.GetChar()
       Dim lastChar As SnapshotPoint = If(CInt(currentChar__1) = 0, currentChar__1, currentChar__1 - 1)
       'if currentChar is 0 (beginning of buffer), don't move it back
       Dim lastText As Char = lastChar.GetChar()
       Dim pairSpan As New SnapshotSpan()
   
       If m_braceList.ContainsKey(currentText) Then
           'the key is the open brace
           Dim closeChar As Char
           m_braceList.TryGetValue(currentText, closeChar)
           If BraceMatchingTagger.FindMatchingCloseChar(currentChar__1, currentText, closeChar, View.TextViewLines.Count, pairSpan) = True Then
               Exit Function
           End If
       ElseIf m_braceList.ContainsValue(lastText) Then
           'the value is the close brace, which is the *previous* character 
           Dim open = From n In m_braceList _
               Where n.Value.Equals(lastText) _
               Select n.Key
           If BraceMatchingTagger.FindMatchingOpenChar(lastChar, CChar(open.ElementAt(0)), lastText, View.TextViewLines.Count, pairSpan) = True Then
               Exit Function
           End If
       End If
   End Function
   

9. The following private methods find the matching brace at any level of nesting. The first method finds the close character that matches the open character:

   private static bool FindMatchingCloseChar(SnapshotPoint startPoint, char open, char close, int maxLines, out SnapshotSpan pairSpan)
   {
       pairSpan = new SnapshotSpan(startPoint.Snapshot, 1, 1);
       ITextSnapshotLine line = startPoint.GetContainingLine();
       string lineText = line.GetText();
       int lineNumber = line.LineNumber;
       int offset = startPoint.Position - line.Start.Position + 1;
   
       int stopLineNumber = startPoint.Snapshot.LineCount - 1;
       if (maxLines > 0)
           stopLineNumber = Math.Min(stopLineNumber, lineNumber + maxLines);
   
       int openCount = 0;
       while (true)
       {
           //walk the entire line
           while (offset < line.Length)
           {
               char currentChar = lineText[offset];
               if (currentChar == close) //found the close character
               {
                   if (openCount > 0)
                   {
                       openCount--;
                   }
                   else    //found the matching close
                   {
                       pairSpan = new SnapshotSpan(startPoint.Snapshot, line.Start + offset, 1);
                       return true;
                   }
               }
               else if (currentChar == open) // this is another open
               {
                   openCount++;
               }
               offset++;
           }
   
           //move on to the next line
           if (++lineNumber > stopLineNumber)
               break;
   
           line = line.Snapshot.GetLineFromLineNumber(lineNumber);
           lineText = line.GetText();
           offset = 0;
       }
   
       return false;
   }
   

   Private Shared Function FindMatchingCloseChar(ByVal startPoint As SnapshotPoint, ByVal open As Char, ByVal close As Char, ByVal maxLines As Integer, ByRef pairSpan As SnapshotSpan) As Boolean
       pairSpan = New SnapshotSpan(startPoint.Snapshot, 1, 1)
       Dim line As ITextSnapshotLine = startPoint.GetContainingLine()
       Dim lineText As String = line.GetText()
       Dim lineNumber As Integer = line.LineNumber
       Dim offset As Integer = startPoint.Position - line.Start.Position + 1
   
       Dim stopLineNumber As Integer = startPoint.Snapshot.LineCount - 1
       If maxLines > 0 Then
           stopLineNumber = Math.Min(stopLineNumber, lineNumber + maxLines)
       End If
   
       Dim openCount As Integer = 0
       While True
           'walk the entire line
           While offset < line.Length
               Dim currentChar As Char = lineText(offset)
               If currentChar = close Then
                   'found the close character
                   If openCount > 0 Then
                       openCount -= 1
                   Else
                       'found the matching close
                       pairSpan = New SnapshotSpan(startPoint.Snapshot, line.Start + offset, 1)
                       Return True
                   End If
               ElseIf currentChar = open Then
                   ' this is another open
                   openCount += 1
               End If
               offset += 1
           End While
   
           'move on to the next line
           If System.Threading.Interlocked.Increment(lineNumber) > stopLineNumber Then
               Exit While
           End If
   
           line = line.Snapshot.GetLineFromLineNumber(lineNumber)
           lineText = line.GetText()
           offset = 0
       End While
   
       Return False
   End Function
   

10. The following helper method finds the open character that matches a close character:

    private static bool FindMatchingOpenChar(SnapshotPoint startPoint, char open, char close, int maxLines, out SnapshotSpan pairSpan)
    {
        pairSpan = new SnapshotSpan(startPoint, startPoint);
    
        ITextSnapshotLine line = startPoint.GetContainingLine();
    
        int lineNumber = line.LineNumber;
        int offset = startPoint - line.Start - 1; //move the offset to the character before this one
    
        //if the offset is negative, move to the previous line
        if (offset < 0)
        {
            line = line.Snapshot.GetLineFromLineNumber(--lineNumber);
            offset = line.Length - 1;
        }
    
        string lineText = line.GetText();
    
        int stopLineNumber = 0;
        if (maxLines > 0)
            stopLineNumber = Math.Max(stopLineNumber, lineNumber - maxLines);
    
        int closeCount = 0;
    
        while (true)
        {
            // Walk the entire line
            while (offset >= 0)
            {
                char currentChar = lineText[offset];
    
                if (currentChar == open)
                {
                    if (closeCount > 0)
                    {
                        closeCount--;
                    }
                    else // We've found the open character
                    {
                        pairSpan = new SnapshotSpan(line.Start + offset, 1); //we just want the character itself
                        return true;
                    }
                }
                else if (currentChar == close)
                {
                    closeCount++;
                }
                offset--;
            }
    
            // Move to the previous line
            if (--lineNumber < stopLineNumber)
                break;
    
            line = line.Snapshot.GetLineFromLineNumber(lineNumber);
            lineText = line.GetText();
            offset = line.Length - 1;
        }
        return false;
    }
    

    Private Shared Function FindMatchingOpenChar(ByVal startPoint As SnapshotPoint, ByVal open As Char, ByVal close As Char, ByVal maxLines As Integer, ByRef pairSpan As SnapshotSpan) As Boolean
        pairSpan = New SnapshotSpan(startPoint, startPoint)
    
        Dim line As ITextSnapshotLine = startPoint.GetContainingLine()
    
        Dim lineNumber As Integer = line.LineNumber
        Dim offset As Integer = startPoint - line.Start - 1
        'move the offset to the character before this one
        'if the offset is negative, move to the previous line
        If offset < 0 Then
            line = line.Snapshot.GetLineFromLineNumber(System.Threading.Interlocked.Decrement(lineNumber))
            offset = line.Length - 1
        End If
    
        Dim lineText As String = line.GetText()
    
        Dim stopLineNumber As Integer = 0
        If maxLines > 0 Then
            stopLineNumber = Math.Max(stopLineNumber, lineNumber - maxLines)
        End If
    
        Dim closeCount As Integer = 0
    
        While True
            ' Walk the entire line
            While offset >= 0
                Dim currentChar As Char = lineText(offset)
    
                If currentChar = open Then
                    If closeCount > 0 Then
                        closeCount -= 1
                    Else
                        ' We've found the open character
                        pairSpan = New SnapshotSpan(line.Start + offset, 1)
                        'we just want the character itself
                        Return True
                    End If
                ElseIf currentChar = close Then
                    closeCount += 1
                End If
                offset -= 1
            End While
    
            ' Move to the previous line
            If System.Threading.Interlocked.Decrement(lineNumber) < stopLineNumber Then
                Exit While
            End If
    
            line = line.Snapshot.GetLineFromLineNumber(lineNumber)
            lineText = line.GetText()
            offset = line.Length - 1
        End While
        Return False
    End Function
    

Implementing a Brace Matching Tagger Provider

In addition to implementing a tagger, you must also implement and export a tagger provider. In this case, the content type of the provider is "text". This means that brace matching will appear in all types of text files, but a fuller implementation would apply brace matching only to a specific content type.

To implement a brace matching tagger provider

1. Declare a tagger provider that inherits from IViewTaggerProvider, name it BraceMatchingTaggerProvider, and export it with a ContentTypeAttribute of "text" and a TagTypeAttribute of TextMarkerTag.

   [Export(typeof(IViewTaggerProvider))]
   [ContentType("text")]
   [TagType(typeof(TextMarkerTag))]
   internal class BraceMatchingTaggerProvider : IViewTaggerProvider
   

   <Export(GetType(IViewTaggerProvider))> _
   <ContentType("text")> _
   <TagType(GetType(TextMarkerTag))> _
   Friend Class BraceMatchingTaggerProvider
       Implements IViewTaggerProvider
   

2. Implement the CreateTagger method to instantiate a BraceMatchingTagger.

   public ITagger<T> CreateTagger<T>(ITextView textView, ITextBuffer buffer) where T : ITag
   {
       if (textView == null)
           return null;
   
       //provide highlighting only on the top-level buffer
       if (textView.TextBuffer != buffer)
           return null;
   
       return new BraceMatchingTagger(textView, buffer) as ITagger<T>;
   }
   

   Public Function CreateTagger(Of T As ITag)(ByVal textView As ITextView, ByVal buffer As ITextBuffer) As ITagger(Of T) Implements IViewTaggerProvider.CreateTagger
       If textView Is Nothing Then
           Return Nothing
       End If
   
       'provide highlighting only on the top-level buffer
       If textView.TextBuffer IsNot buffer Then
           Return Nothing
       End If
   
       Return TryCast(New BraceMatchingTagger(textView, buffer), ITagger(Of T))
   End Function
   

Building and Testing the Code

To test this code, build the BraceMatchingTest solution and run it in the experimental instance.

To build and test BraceMatchingTest solution

1. Build the solution.

2. When you run this project in the debugger, a second instance of Visual Studio is instantiated.

3. Create a text file and type some text that includes matching braces.

   hello {  
   goodbye}  
   
   {}  
   
   {hello}  
   

4. When you position the caret before an open brace, both that brace and the matching close brace should be highlighted. When you position the cursor just after the close brace, both that brace and the matching open brace should be highlighted.

See Also

Walkthrough: Linking a Content Type to a File Name Extension