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IComparable.CompareTo Method

Updated: January 2011

Compares the current instance with another object of the same type and returns an integer that indicates whether the current instance precedes, follows, or occurs in the same position in the sort order as the other object.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
int CompareTo(
	Object obj
)

Parameters

obj
Type: System.Object

An object to compare with this instance.

Return Value

Type: System.Int32
A 32-bit signed integer that indicates the relative order of the objects being compared. The return value has these meanings:

Value

Meaning

Less than zero

This instance precedes obj in the sort order.

Zero

This instance occurs in the same position in the sort order as obj.

Greater than zero

This instance follows obj in the sort order.

ExceptionCondition
ArgumentException

obj is not the same type as this instance.

The CompareTo method is implemented by types whose values can be ordered or sorted. It is called automatically by methods of non-generic collection objects, such as Array.Sort, to order each member of the array. If a custom class or structure does not implement IComparable, its members cannot be ordered and the sort operation can throw an InvalidOperationException.

This method is only a definition and must be implemented by a specific class or value type to have effect. The meaning of the comparisons specified in the Return Value section ("precedes", "occurs in the same position as", and "follows") depends on the particular implementation.

By definition, any object compares greater than (or follows) null, and two null references compare equal to each other.

The parameter, obj, must be the same type as the class or value type that implements this interface; otherwise, an ArgumentException is thrown.

Notes to Implementers:

For objects A, B and C, the following must be true:

A.CompareTo(A) is required to return zero.

If A.CompareTo(B) returns zero, then B.CompareTo(A) is required to return zero.

If A.CompareTo(B) returns zero and B.CompareTo(C) returns zero, then A.CompareTo(C) is required to return zero.

If A.CompareTo(B) returns a value other than zero, then B.CompareTo(A) is required to return a value of the opposite sign.

If A.CompareTo(B) returns a value x not equal to zero, and B.CompareTo(C) returns a value y of the same sign as x, then A.CompareTo(C) is required to return a value of the same sign as x and y.

Notes to Callers:

Use the CompareTo method to determine the ordering of instances of a class.

The following code sample illustrates the use of CompareTo to compare a Temperature object implementing IComparable with another object. The Temperature object implements CompareTo by simply wrapping a call to the Int32.CompareTo method.

using System;
using System.Collections;

public class Temperature : IComparable 
{
    // The temperature value 
    protected double temperatureF;

    public int CompareTo(object obj) {
        if (obj == null) return 1;

        Temperature otherTemperature = obj as Temperature;
        if (otherTemperature != null) 
            return this.temperatureF.CompareTo(otherTemperature.temperatureF);
        else 
           throw new ArgumentException("Object is not a Temperature");
    }

    public double Fahrenheit 
    {
        get 
        {
            return this.temperatureF;
        }
        set {
            this.temperatureF = value;
        }
    }

    public double Celsius 
    {
        get 
        {
            return (this.temperatureF - 32) * (5.0/9);
        }
        set 
        {
            this.temperatureF = (value * 9.0/5) + 32;
        }
    }
}

public class CompareTemperatures
{
   public static void Main()
   {
      ArrayList temperatures = new ArrayList();
      // Initialize random number generator.
      Random rnd = new Random();

      // Generate 10 temperatures between 0 and 100 randomly. 
      for (int ctr = 1; ctr <= 10; ctr++)
      {
         int degrees = rnd.Next(0, 100);
         Temperature temp = new Temperature();
         temp.Fahrenheit = degrees;
         temperatures.Add(temp);   
      }

      // Sort ArrayList.
      temperatures.Sort();

      foreach (Temperature temp in temperatures)
         Console.WriteLine(temp.Fahrenheit);

   }
}
// The example displays the following output to the console (individual 
// values may vary because they are randomly generated): 
//       2 
//       7 
//       16 
//       17 
//       31 
//       37 
//       58 
//       66 
//       72 
//       95

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

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

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

January 2011

Revised the Return Value and Remarks sections.

Customer feedback.

July 2008

Added detail on usage by the common language runtime.

Customer feedback.

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