User-defined conversion operators (C# reference)

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A user-defined type can define a custom implicit or explicit conversion from or to another type.

Implicit conversions don't require special syntax to be invoked and can occur in a variety of situations, for example, in assignments and methods invocations. Predefined C# implicit conversions always succeed and never throw an exception. User-defined implicit conversions should behave in that way as well. If a custom conversion can throw an exception or lose information, define it as an explicit conversion.

User-defined conversions are not considered by the is and as operators. Use a cast expression to invoke a user-defined explicit conversion.

Use the operator and implicit or explicit keywords to define an implicit or explicit conversion, respectively. The type that defines a conversion must be either a source type or a target type of that conversion. A conversion between two user-defined types can be defined in either of the two types.

The following example demonstrates how to define an implicit and explicit conversion:

using System;

public readonly struct Digit
{
    private readonly byte digit;

    public Digit(byte digit)
    {
        if (digit > 9)
        {
            throw new ArgumentOutOfRangeException(nameof(digit), "Digit cannot be greater than nine.");
        }
        this.digit = digit;
    }

    public static implicit operator byte(Digit d) => d.digit;
    public static explicit operator Digit(byte b) => new Digit(b);

    public override string ToString() => $"{digit}";
}

public static class UserDefinedConversions
{
    public static void Main()
    {
        var d = new Digit(7);

        byte number = d;
        Console.WriteLine(number);  // output: 7

        Digit digit = (Digit)number;
        Console.WriteLine(digit);  // output: 7
    }
}

You also use the operator keyword to overload a predefined C# operator. For more information, see Operator overloading.

C# language specification

For more information, see the following sections of the C# language specification:

See also