IComparable<T> Interface


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Defines a generalized comparison method that a value type or class implements to create a type-specific comparison method for ordering or sorting its instances.

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

generic<typename T>
public interface class IComparable

Type Parameters


The type of object to compare.


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.

This interface is implemented by types whose values can be ordered or sorted and provides a strongly typed comparison method for ordering members of a generic collection object. For example, one number can be larger than a second number, and one string can appear in alphabetical order before another. It requires that implementing types define a single method, CompareTo(T), that indicates whether the position of the current instance in the sort order is before, after, or the same as a second object of the same type. Typically, the method is not called directly from developer code. Instead, it is called automatically by methods such as List<T>::Sort() and Add.

Typically, types that provide an IComparable<T> implementation also implement the IEquatable<T> interface. The IEquatable<T> interface defines the Equals method, which determines the equality of instances of the implementing type.

The implementation of the CompareTo(T) method must return an Int32 that has one of three values, as shown in the following table.



Less than zero

This object precedes the object specified by the CompareTo method in the sort order.


This current instance occurs in the same position in the sort order as the object specified by the CompareTo method argument.

Greater than zero

This current instance follows the object specified by the CompareTo method argument in the sort order.

All numeric types (such as Int32 and Double) implement IComparable<T>, as do String, Char, and DateTime. Custom types should also provide their own implementation of IComparable<T> to enable object instances to be ordered or sorted.

Notes to Implementers:

Replace the type parameter of the IComparable<T> interface with the type that is implementing this interface.

If you implement IComparable<T>, you should overload the op_GreaterThan, op_GreaterThanOrEqual, op_LessThan, and op_LessThanOrEqual operators to return values that are consistent with CompareTo. In addition, you should also implement IEquatable<T>. See the IEquatable<T> article for complete information.

The following code example illustrates the implementation of IComparable<T> 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.dll>

using namespace System;
using namespace System::Collections::Generic;

public ref class Temperature: public IComparable<Temperature^> {

   // The underlying temperature value.
   Double m_value;

   // Implement the generic CompareTo method with the Temperature class 
   // as the Type parameter. 
   virtual Int32 CompareTo( Temperature^ other ) {

      // If other is not a valid object reference, this instance 
      // is greater.
      if (other == nullptr) return 1;

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

       // Define the is greater than operator.
    bool operator>=  (Temperature^ other)
       return CompareTo(other) == 1;

    // Define the is less than operator.
    bool operator<  (Temperature^ other)
       return CompareTo(other) == -1;

       // Define the is greater than or equal to operator.
    bool operator>  (Temperature^ other)
       return CompareTo(other) >= 0;

    // Define the is less than or equal to operator.
    bool operator<=  (Temperature^ other)
       return CompareTo(other) <= 0;

   property Double Celsius {
      Double get() {
         return m_value + 273.15;

   property Double Kelvin {
      Double get() {
         return m_value;
      void set( Double value ) {
         if (value < 0)
            throw gcnew ArgumentException("Temperature cannot be less than absolute zero.");
            m_value = value;

   Temperature(Double kelvins) {
      this->Kelvin = kelvins;

int main() {
   SortedList<Temperature^, String^>^ temps = 
      gcnew SortedList<Temperature^, String^>();

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

   for each( KeyValuePair<Temperature^, String^>^ kvp in temps )
      Console::WriteLine("{0} is {1} degrees Celsius.", kvp->Value, kvp->Key->Celsius);
/* The example displays the following output:
      Absolute zero is 273.15 degrees Celsius.
      Freezing point of water is 546.3 degrees Celsius.
      Boiling point of water is 646.3 degrees Celsius.
      Melting point of Lead is 873.8 degrees Celsius.
      Boiling point of Lead is 2290.3 degrees Celsius.
      Boiling point of Carbon is 5373.3 degrees Celsius.

Universal Windows Platform
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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