Object.ToString Method (System)

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Microsoft Visual Studio 2010/.NET Framework 4

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.NET Framework Class Library

Object..::.ToString Method

Updated: March 2011

Returns a string that represents the current object.

Namespace: System
Assembly: mscorlib (in mscorlib.dll)

Syntax

Visual Basic

Public Overridable Function ToString As String

public virtual string ToString()

Visual C++

public:
virtual String^ ToString()

abstract ToString : unit -> string 
override ToString : unit -> string

Return Value

Type: System..::.String
A string that represents the current object.

Remarks

ToString is the major formatting method in the .NET Framework. It converts an object to its string representation so that it is suitable for display. (For information about formatting support in the .NET Framework, see Formatting Types.)

The default implementation of the ToString method returns the fully qualified name of the type of the Object, as the following example shows.

Visual Basic

Module Example
   Public Sub Main()
      Dim obj As New Object()
      Console.WriteLine(obj.ToString())
   End Sub
End Module
' The example displays the following output:
'      System.Object

using System;

public class Example
{
   public static void Main()
   {
      Object obj = new Object();
      Console.WriteLine(obj.ToString());
   }
}
// The example displays the following output:
//      System.Object

Because Object is the base class of all reference types in the .NET Framework, this behavior is inherited by reference types that do not override the ToString method. The following example illustrates this. It defines a class named Object1 that accepts the default implementation of all Object members. Its ToString method returns the object's fully qualified type name.

Visual Basic

Imports Examples

Namespace Examples
   Public Class Object1
   End Class
End Namespace

Module Example
   Public Sub Main()
      Dim obj1 As New Object1()
      Console.WriteLine(obj1.ToString())
   End Sub
End Module
' The example displays the following output:
'   Examples.Object1

using System;
using Examples;

namespace Examples
{
   public class Object1
   {
   }
}

public class Example
{
   public static void Main()
   {
      object obj1 = new Object1();
      Console.WriteLine(obj1.ToString());
   }
}
// The example displays the following output:
//   Examples.Object1

Types commonly override the ToString method to return a string that represents the object instance. For example, the base types such as Char, Int32, and String provide ToString implementations that return the string form of the value that the object represents. The following example defines a class, Object2, that overrides the ToString method to return the type name along with its value.

Visual Basic

Public Class Object2
   Private value As Object

   Public Sub New(value As Object)
      Me.value = value
   End Sub

   Public Overrides Function ToString() As String
      Return MyBase.ToString + ": " + value.ToString()
   End Function
End Class

Module Example
   Public Sub Main()
      Dim obj2 As New Object2("a"c)
      Console.WriteLine(obj2.ToString())
   End Sub
End Module
' The example displays the following output:
'       Object2: a

using System;

public class Object2
{
   private object value;

   public Object2(object value)
   {
      this.value = value;
   }

   public override string ToString()
   {
      return base.ToString() + ": " + value.ToString();
   }
}

public class Example
{
   public static void Main()
   {
      Object2 obj2 = new Object2('a');
      Console.WriteLine(obj2.ToString());
   }
}
// The example displays the following output:
//       Object2: a

Notes to Implementers

When you implement your own types, you should override the ToString method to return values that are meaningful for those types. Derived classes that require more control over formatting than ToString provides can implement the IFormattable interface. Its IFormattable..::.ToString(String, IFormatProvider) method enables you to define format strings that control formatting and to use an IFormatProvider object that can provide for culture-specific formatting.

Version Information

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Platforms

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference

Object Class

System Namespace

Other Resources

Formatting Types

Change History

Date	History	Reason
March 2011	Revised extensively.	Customer feedback.

Date

History

Reason

March 2011

Revised extensively.

Customer feedback.

ToString Returning a Null Reference

While the practice of overriding the ToString method to return a null reference is not prohibited, I would strongly discourage it.

The ToString method is designed to return a meaningful string representation of an object, and one that serves to identify the object. Where possible, it should return a string representation of the object's value (as is the case with strings, numbers, and dates). If that's not possible or desireable, it should return a string indicating the object's type (as is the case with collection objects, for example, or with the default implementation of the Object.ToString method).

For formatting operations, there's no particular harm in returning a null reference, since an attempt to use the returned null string in a formatting operation simply produces an empty string. For example, If null_String is a null string, the statement Console.WriteLine(nullString) results in an empty string being displayed to the console. Similarly, the statement String.Format("{0}", null_String) returns an empty string.

The major problem arises when attempting to call instance methods on the null string, since these method calls will always throw a NullReferenceException. And since a string returned by a call to an object's ToString method is assumed to be valid (that is, non-null), it's unlikely that developers who consume the type will check to see whether the string returned by ToString is non-null before calling its instance methods.

I hope that this helps to answer your question.

--Ron Petrusha
Common Language Runtime User Education
Microsoft Corporation