CA2241: Provide correct arguments to formatting methods

Cause

The format string argument passed to a method such as WriteLine, Write, or System.String.Format does not contain a format item that corresponds to each object argument, or vice versa.

Rule Description

The arguments to methods such as WriteLine, Write, and Format consist of a format string followed by several System.Object instances. The format string consists of text and embedded format items of the form, {index[,alignment][:formatString]}. 'index' is a zero-based integer that indicates which of the objects to format. If an object does not have a corresponding index in the format string, the object is ignored. If the object specified by 'index' does not exist, a System.FormatException is thrown at runtime.

How to Fix Violations

To fix a violation of this rule, provide a format item for each object argument and provide an object argument for each format item.

When to Suppress Warnings

Do not suppress a warning from this rule.

Example

The following example shows two violations of the rule.

using System;

namespace UsageLibrary
{
   class CallsStringFormat
   {
      void CallFormat()
      {
         string file = "file name";
         int errors = 13;

         // Violates the rule.
         Console.WriteLine(string.Format("{0}", file, errors));

         Console.WriteLine(string.Format("{0}: {1}", file, errors));

         // Violates the rule and generates a FormatException at runtime.
         Console.WriteLine(string.Format("{0}: {1}, {2}", file, errors));
      }
   }
}

Imports System

Namespace UsageLibrary

   Class CallsStringFormat

      Sub CallFormat()

         Dim file As String = "file name"
         Dim errors As Integer = 13

         ' Violates the rule.
         Console.WriteLine(String.Format("{0}", file, errors))

         Console.WriteLine(String.Format("{0}: {1}", file, errors))

         ' Violates the rule and generates a FormatException at runtime.
         Console.WriteLine(String.Format("{0}: {1}, {2}", file, errors))

      End Sub

   End Class

End Namespace