CompareTo Method
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IComparable<T>::CompareTo Method (T)

 

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.dll>

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

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

protected:
   // The underlying temperature value.
   Double m_value;

public:
   // 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.");
         else
            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 4.5
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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