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Because C# is statically-typed at compile time, after a variable is declared, it cannot be declared again or assigned a value of another type unless that type is implicitly convertible to the variable's type. For example, the string cannot be implicitly converted to int . Therefore, after you declare i as an int , you cannot assign the string "Hello" to it, as the following code shows:

int i;
// error CS0029: Cannot implicitly convert type 'string' to 'int'
i = "Hello";
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:
Implicit conversions: No special syntax is required because the conversion always succeeds and no data will be lost. Examples include conversions from smaller to larger integral types, and conversions from derived classes to base classes.
Explicit conversions (casts): Explicit conversions require a cast expression. Casting is required when information might be lost in the conversion, or when the conversion might not succeed for other reasons. Typical examples include numeric conversion to a type that has less precision or a smaller range, and conversion of a base-class instance to a derived class.
User-defined conversions: User-defined conversions are performed by special methods that you can define to enable explicit and implicit conversions between custom types that do not have a base class–derived class relationship. For more information, see User-defined conversion operators.
Conversions with helper classes: To convert between non-compatible types, such as integers and System.DateTime objects, or hexadecimal strings and byte arrays, you can use the System.BitConverter class, the System.Convert class, and the Parse methods of the built-in numeric types, such as Int32.Parse. For more information, see How to convert a byte array to an int, How to convert a string to a number, and How to convert between hexadecimal strings and numeric types.
Implicit 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 integral types, this means the range of the source type is a proper subset of the range for the target type. For example, a variable of type long (64-bit integer) can store any value that an int (32-bit integer) can store. In the following example, the compiler implicitly converts the value of num on the right to a type long before assigning it to bigNum.
// Implicit conversion. A 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 the Implicit numeric conversions section of the Built-in numeric conversions article.
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();
// Always OK.
Base b = d;
Explicit conversions
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, or the cast may fail at run time. 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 complete list of supported explicit numeric conversions, see the Explicit numeric conversions section of the Built-in numeric conversions article.
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.
Type conversion exceptions at run time
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() => System.Console.WriteLine("Eating.");
    public override string ToString() => "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)
        // System.InvalidCastException at run time
        // Unable to cast object of type 'Mammal' to type 'Reptile'
        Reptile r = (Reptile)a;
The Test method has an Animal parameter, thus explicitly casting the argument a to a Reptile makes a dangerous assumption. It is safer to not make assumptions, but rather check the type. C# provides the is operator to enable you to test for compatibility before actually performing a cast. For more information, see How to safely cast using pattern matching and the as and is operators.
C# language specification
For more information, see the Conversions section of the C# language specification.
See also
C# Programming Guide
Types
Cast expression
User-defined conversion operators
Generalized Type Conversion
How to convert a string to a number