Generate a const

[adamconnelly]

Thrift supports defining constants that will be generated into C#, and can also be referenced within the Thrift IDL. Here's an example:

const i32 MagicNumber = 123

struct Test {
  1: i32 Field = MagicNumber
}

This would generate something along these lines:

public static class Constants
{
    public const int MagicNumber = 123;
}

public class Test
{
    private int? _Field = Constants.MagicNumber;

    public int? Field
    {
        get
        {
            return _Field;
        }

        set
        {
            _Field = value;
        }
    }
}

The Apache compiler generates a file for the constants called <thrift-filename>.Constants.cs, so if we have a file called User.thrift, we'll get a User.Constants.cs file produced with a UserConstants class inside it.

At the moment we've taken the approach of generating all of the definitions in a single .thrift file into a single .cs file with the same name. So a User.thrift file will cause a User.cs file to be created, rather than creating files named based on the thrift definition names. We'll aim to continue with this approach, but use the Thrift filename to create the name of the constants class. If the filename is called Constants.thrift, we might want to provide a special case and avoid duplicating the word Constants.

So we might end up with the following class names:

• User.thrift -> UserConstants.
• Constants.thrift -> Constants.
• My.File.thrift -> My_FileConstants.

Questions:

• Do we want to be smart about casing (e.g. constants.thrift -> Constants, user.thrift -> UserConstants)?

Tasks:

• Support int constants.
• Support decimal constants.
• Support bool constants.
• Support string constants.
• Support collection constants.
• Support custom object constants.
• Initialise from referenced constant.

Errors / warnings:

• Invalid type conversion (string assigned to an int, etc).
• Value outside allowed range for type.
• Name missing.
• Deprecation warning for declaring a constant using slist.

Notes:

• The official compiler doesn't seem to check the range of the constant expression is valid, so allows const i8 MyI8 = 128, for example.
• The official compiler accepts true, false, and integers for bool constants, where <= 0 is false and positive numbers are true.