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Casting and Type Conversions (C# Programming Guide)

Because C# is statically-typed at compile time, after a variable is declared, it cannot be declared again or used to store values of another type unless that type is convertible to the variable's type. For example, there is no conversion from an integer to any arbitrary string. Therefore, after you declare i as an integer, you cannot assign the string "Hello" to it, as is shown in the following code.

int i;
i = "Hello"; // Error: "Cannot implicitly convert type 'string' to 'int'"

However, you might sometimes need to copy a value into a variable or method parameter of another type. For example, you might have an integer variable that you need to pass to a method whose parameter is typed as double. Or you might need to assign a class variable to a variable of an interface type. These kinds of operations are called type conversions. In C#, you can perform the following kinds of conversions:

For built-in numeric types, an implicit conversion can be made when the value to be stored can fit into the variable without being truncated or rounded off. For example, a variable of type long (8 byte integer) can store any value that an int (4 bytes on a 32-bit computer) can store. In the following example, the compiler implicitly converts the value on the right to a type long before assigning it to bigNum.


// Implicit conversion. num long can
// hold any value an int can hold, and more!
int num = 2147483647;
long bigNum = num;


For a complete list of all implicit numeric conversions, see Implicit Numeric Conversions Table (C# Reference).

For reference types, an implicit conversion always exists from a class to any one of its direct or indirect base classes or interfaces. No special syntax is necessary because a derived class always contains all the members of a base class.

Derived d = new Derived();
Base b = d; // Always OK.

However, if a conversion cannot be made without a risk of losing information, the compiler requires that you perform an explicit conversion, which is called a cast. A cast is a way of explicitly informing the compiler that you intend to make the conversion and that you are aware that data loss might occur. To perform a cast, specify the type that you are casting to in parentheses in front of the value or variable to be converted. The following program casts a double to an int. The program will not compile without the cast.


class Test
{
    static void Main()
    {
        double x = 1234.7;
        int a;
        // Cast double to int.
        a = (int)x;
        System.Console.WriteLine(a);
    }
}
// Output: 1234


For a list of the explicit numeric conversions that are allowed, see Explicit Numeric Conversions Table (C# Reference).

For reference types, an explicit cast is required if you need to convert from a base type to a derived type:

// Create a new derived type.
Giraffe g = new Giraffe();

// Implicit conversion to base type is safe.
Animal a = g;

// Explicit conversion is required to cast back
// to derived type. Note: This will compile but will
// throw an exception at run time if the right-side
// object is not in fact a Giraffe.
Giraffe g2 = (Giraffe) a;

A cast operation between reference types does not change the run-time type of the underlying object; it only changes the type of the value that is being used as a reference to that object. For more information, see Polymorphism (C# Programming Guide).

In some reference type conversions, the compiler cannot determine whether a cast will be valid. It is possible for a cast operation that compiles correctly to fail at run time. As shown in the following example, a type cast that fails at run time will cause an InvalidCastException to be thrown.



class Animal
{
    public void Eat() { Console.WriteLine("Eating."); }
    public override string ToString()
    {
        return "I am an animal.";
    }
}
class Reptile : Animal { }
class Mammal : Animal { }

class UnSafeCast
{
    static void Main()
    {            
        Test(new Mammal());

        // Keep the console window open in debug mode.
        System.Console.WriteLine("Press any key to exit.");
        System.Console.ReadKey();
    }

    static void Test(Animal a)
    {
        // Cause InvalidCastException at run time 
        // because Mammal is not convertible to Reptile.
        Reptile r = (Reptile)a;
    }

}


C# provides the is and as operators to enable you to test for compatibility before actually performing a cast. For more information, see How to: Safely Cast by Using as and is Operators (C# Programming Guide).

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

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