Runtime: [Api Proposal] Generic Convert.ChangeType

[benaadams]

Rather than boxing back to object and then having to cast to type; it would be good to have a change type that returns the type:

Proposed API

partial class Convert
{
    [return: NotNullIfNotNull("value")]
    public static T? ChangeType<T>(object? value);

    [return: NotNullIfNotNull("value")]
    public static T? ChangeType<T>(object? value, IFormatProvider? provider);
}

Questions

Not sure on nullable return type for structs?

[EgorBo]

If we make object? ChangeType(object? value, TypeCode typeCode) inlineable Jit should be able to eliminate the intermediate boxing I guess. Isn't typeCode a constant in most use-cases?

It's not the first time I am thinking of a special attribute InlineIfConst.. I know JIT takes constant input into account for inlining budget but it won't help with large methods.

[benaadams]

public static object? ChangeType(object? value, Type conversionType) is more likely the called overload, and then it has to do a look up to get the typecode falling through a bunch of if tests.

[benaadams]

Then when you use it in the generic as well as reboxing the output you then have to cast it to the type
e..g

T value = (T)Convert.ChangeType(inputValue, typeof(T));

[EgorBo]

[then it has to do a look up to get the typecode](https://github.com/dotnet/runtime/blob/3b72a4d7013abd36b0f2e145f44df85c43fc83aa/src/libraries/System.Private.CoreLib/src/System/Convert.cs#L287-L352) falling through a bunch ofif` tests.

yeah but if typecode is a const -- all those ifs will be easily eliminated 🙂

[EgorBo]

Then when you use it in the generic as well as reboxing the output you then have to cast it to the type
e..g

T value = (T)Convert.ChangeType(inputValue, typeof(T));

Jit eliminates boxing for (T)(object)varOfT, see https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AXEBDAzgWwB8ABAJgEYBYAKGIAYACAZQAtsoAHAGW2ADoASgFcAdhgCW+GAG4aNYgGYGZBgGEGAbxoMdDbboDaAKXEYA4jBEwo4sAAoMATw4wIAMzvixASjQMnLu6ePt4Auvo6igwAKgAiuBgMALIAPNFMUGB+cQkAfHbpmf7eEZqlusoA7AwF8RjedhDAAFYwYPUYstQVAL40PUA=

[benaadams]

As a developer/user would I prefer to write:

T value = (T)Convert.ChangeType(inputValue, typeof(T));

Or

T value = Convert.ChangeType<T>(inputValue);

An the implementation of the second done via generics could be more efficient than the first using type?