IConvertible Interface

Aug 26, 2008
by David Cumps

Part of my 2008 study notes for exam 70-536 (.NET Framework 2.0).

Overview

Enables a class to be converted to a base type such as Boolean, Byte, Double, or String.
C# allows widening conversion without any casting, e.g. going from int to double, see below.
Narrowing conversion requires explicit conversion.
Overloading operators allows for implicit and explicit conversion of custom types, as demonstrated below.
Implementing IConvertible allows the use of Convert with your custom type.

Examples

Widening Conversion

int i = 1;
double d = 1.0001;
d = i; // Compiles without casting.

Narrowing Conversion

Int16 a = 1;
Int32 b = 1;
double c = 1;
 
a = (Int16)b; // a = b will not be allowed, narrowing!
a = (Int16)c; // a = c will not be allowed, narrowing!
 
b = a;        // Allowed, widening
b = (Int32)c; // b = c will not be allowed, narrowing!
 
c = a;        // Allowed, widening
c = b;        // Allowed, widening

Overloading Operators

class Program
{
    static void Main(string[] args)
    {
        MyPlaceholder a;
        int b;
 
        // Allowed because of the 'implicit' operator
        a = 5;
        Console.WriteLine(a.Value);
 
        // Allowed because of the string implicit operator,
        // don't do this however! Way too error-prone!
        a = "7";
        Console.WriteLine(a.Value);
 
        // Allowed because of the 'explicit' operator
        b = (int)a;
        Console.WriteLine(b);
    }
}
 
public class MyPlaceholder
{
    public int Value { get; private set; }
 
    public MyPlaceholder(int value)
    {
        Value = value;
    }
 
    public static implicit operator MyPlaceholder(int arg)
    {
        MyPlaceholder obj = new MyPlaceholder(arg);
        return obj;
    }
 
    public static implicit operator MyPlaceholder(string arg)
    {
        MyPlaceholder obj = new MyPlaceholder(Convert.ToInt32(arg));
        return obj;
    }
 
    public static explicit operator int(MyPlaceholder arg)
    {
        return arg.Value;
    }
}

Implementing IConvertible

class Program
{
    static void Main(string[] args)
    {
        MyPlaceholder a = new MyPlaceholder(5);
        int b;
 
        // Allowed because of the IConvertible interface
        b = Convert.ToInt32(a);
        Console.WriteLine(b);
 
        // Allowed because of the IConvertible interface
        Console.WriteLine(Convert.ToDouble(a));
 
        // Not Allowed because of the InvalidCastException
        Console.WriteLine(Convert.ToDateTime(a));
    }
}
 
public class MyPlaceholder: IConvertible
{
    public int Value { get; private set; }
 
    public MyPlaceholder(int value)
    {
        Value = value;
    }
 
    #region IConvertible Members
    public double ToDouble(IFormatProvider provider) { return Value; }
    public float ToSingle(IFormatProvider provider) { return Value; }
    public int ToInt32(IFormatProvider provider) { return Value; }
    public long ToInt64(IFormatProvider provider) { return Value; }
    public string ToString(IFormatProvider provider) { return Value.ToString(); }
 
    public TypeCode GetTypeCode()
    {
        return TypeCode.Object;
    }
 
    public bool ToBoolean(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public byte ToByte(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public char ToChar(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public DateTime ToDateTime(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public decimal ToDecimal(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public short ToInt16(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public sbyte ToSByte(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public object ToType(Type conversionType, IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public ushort ToUInt16(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public uint ToUInt32(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
 
    public ulong ToUInt64(IFormatProvider provider)
    {
        throw new InvalidCastException();
    }
    #endregion
}