是否Convert.ChangeType快捷方式？

Question

I'm wondering what happens inside Object Convert.ChangeType( Object object, Type type) when the type of object matches type . Does it just short-cut and return object ?

For example:

object objString = "Hello World";

string converted = (string) Convert.ChangeType( objString, typeof ( String ) );

Would the implementation of ChangeType actually call the IConvertible interface or just return objString?

Answer 1

According to Microsoft's C# reference source , Convert.ChangeType(Object, Type) performs the following general behaviour:

• If the input is null
  • If the type is a value type, throw.
  • Otherwise return null.
• If the input is not IConvertible
  • If the type is exactly the input's type, return the input.
  • Otherwise throw.
• If the type is one of the out-of-the-box core convertible types, call the corresponding ToWhatever method on the input.
  • In each case, the implementation seems to be return this if the types match or return Convert.ToWhatever(this) otherwise, which is a shortcut of sorts.
• Otherwise call ToType on the input, passing the type through.

Answer 2

Yes, it would call IConvertible interface on the object. In case of string it would call objString.ToString() which in turn would return itself (return this).

In addition, if type of object does not implement IConvertible and you convert to the same type then it will just return the same object.

If type does not implement IConvertible and you convert to different type then exception will be thrown.

Answer 3

This MSDN blog article might help you.

If the class implements IConvertable interface, you can then use System.Convert.ChangeType to change the data type:

decimal x = (decimal) System.Convert.ChangeType("5", typeof(decimal));

Think of ChangeType as a big switch statement (select case in VB)… with lots of overloaded functions. Something like this (disclaimer: this is intended to be a pseudo-code, not the exact .NET's implementation):

public static Object ChangeType(Object value, TypeCode typeCode , IFormatProvider provider)
{

IConvertible v = value as IConvertible;

switch (typeCode) {

case TypeCode.Boolean:
    return v.ToBoolean(provider);

case TypeCode.Char:
    return v.ToChar(provider);

case TypeCode.SByte:
    return v.ToSByte(provider);

case TypeCode.Byte:
    return v.ToByte(provider);

case TypeCode.Int16:
    return v.ToInt16(provider);

case TypeCode.UInt16:
    return v.ToUInt16(provider);

. . .

}

A class that claims to implement IConvertible interface must implement all the conversions in the switch structure above plus GetTypeCode:

· GetTypeCode

· ToBoolean

· ToByte

· ToChar

· ToDateTime

· ToDecimal

· ToDouble

· ToInt16

· ToInt32

· ToInt64

· ToSByte

· ToSingle

· ToString

· ToType

· ToUInt16

· ToUInt32

· ToUInt64

System.Convert class has a number of implementations that can be, in turn, called by the class implementing IConvertible interface.