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CompareTo Method
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IComparable<T>.CompareTo Method

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(
	T other
)

Parameters

other
Type: T

An object to compare with this instance.

Return Value

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

Value

Meaning

Less than zero

This instance precedes other in the sort order.

Zero

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

Greater than zero

This instance follows other in the sort order.

CompareTo provides a strongly typed comparison method for ordering members of a generic collection object. Because of this, it is usually not called directly from developer code. Instead, it is called automatically by methods such as List<T>.Sort() and Add.

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 Values section ("precedes", "occurs in the same position as", and "follows) depends on the particular implementation.

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

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 that is 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 example illustrates the implementation of IComparable for a simple Temperature object. The example creates a SortedList<TKey, TValue> collection of strings with Temperature object keys, and adds several pairs of temperatures and strings to the list out of sequence. In the call to the Add method, the SortedList<TKey, TValue> collection uses the IComparable<T> implementation to sort the list entries, which are then displayed in order of increasing temperature.

using System;
using System.Collections.Generic;

public class Temperature : IComparable<Temperature>
{
    // Implement the generic CompareTo method with the Temperature  
    // class as the Type parameter.  
    // 
    public int CompareTo(Temperature other)
    {
        // If other is not a valid object reference, this instance is greater. 
        if (other == null) return 1;

        // The temperature comparison depends on the comparison of  
        // the underlying Double values.  
        return m_value.CompareTo(other.m_value);
    }

    // The underlying temperature value. 
    protected double m_value = 0.0;

    public double Celsius    
    {
        get
        {
            return m_value - 273.15;
        }
    }

    public double Kelvin    
    {
        get
        {
            return m_value;
        }
        set
        {
            if (value < 0.0)
            {
                throw new ArgumentException("Temperature cannot be less than absolute zero.");
            }
            else
            {
                m_value = value;
            }
        }
    }

    public Temperature(double kelvins)
    {
        this.Kelvin = kelvins;
    }
}

public class Example
{
    public static void Main()
    {
        SortedList<Temperature, string> temps = 
            new SortedList<Temperature, string>();

        // Add entries to the sorted list, out of order.
        temps.Add(new Temperature(2017.15), "Boiling point of Lead");
        temps.Add(new Temperature(0), "Absolute zero");
        temps.Add(new Temperature(273.15), "Freezing point of water");
        temps.Add(new Temperature(5100.15), "Boiling point of Carbon");
        temps.Add(new Temperature(373.15), "Boiling point of water");
        temps.Add(new Temperature(600.65), "Melting point of Lead");

        foreach( KeyValuePair<Temperature, string> kvp in temps )
        {
            Console.WriteLine("{0} is {1} degrees Celsius.", kvp.Value, kvp.Key.Celsius);
        }
    }
}
/* This example displays the following output:
      Absolute zero is -273.15 degrees Celsius.
      Freezing point of water is 0 degrees Celsius.
      Boiling point of water is 100 degrees Celsius.
      Melting point of Lead is 327.5 degrees Celsius.
      Boiling point of Lead is 1744 degrees Celsius.
      Boiling point of Carbon is 4827 degrees Celsius.
*/

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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