Day 8: C# Casting and type conversions

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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.

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.

Implicit conversions
Explicit conversions (casts)
User-defined conversions
Conversions with helper classes

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.

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

Derived d = new Derived();

// Always OK.
Base b = d;

char -> int -> long -> float -> double

Explicit conversions (casts)

However, if a conversion cannot be made without 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.

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

double -> float -> long -> int -> char

Invalid Casting Error

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;
    }
}

Type Conversion Methods

It is also possible to convert data types explicitly by using built-in methods, such as Convert.ToBoolean, Convert.ToDouble, Convert.ToString, Convert.ToInt32 (int) and Convert.ToInt64 (long):

int myInt = 1023;
double myDouble = 8.12;
bool myBool = false;

Console.WriteLine(Convert.ToString(myInt));    // convert int to string
Console.WriteLine(Convert.ToDouble(myInt));    // convert int to double
Console.WriteLine(Convert.ToInt32(myDouble));  // convert double to int
Console.WriteLine(Convert.ToString(myBool));   // convert bool to string