FormUrlEncodedJson calls wrong overload of Error.Argument

src/System.Net.Http.Formatting/Formatting/FormUrlEncodedJson.cs から始め、特に FormUrlEncodedJson.AddToArray を確認し、その Error.Argument 呼び出しを src/Common/Error.cs のオーバーロードと比較します。フォーマット文字列とフォーマット引数だけを使用する呼び出しが意図したオーバーロードに解決されることを確認してください。フォーマット文字列がパラメーター名として扱われなくなれば完了です。

src/System.Net.Http.Formatting/Formatting/FormUrlEncodedJson.cs contains several Error.Argument calls that do not specify the parameter name. For example, in the FormUrlEncodedJson.AddToArray method:

https://github.com/aspnet/AspNetWebStack/blob/1231b77d79956152831b75ad7f094f844251b97f/src/System.Net.Http.Formatting/Formatting/FormUrlEncodedJson.cs#L400-L404

The arguments in this call are a format string from resources, and a string to use as a format argument. The call is intended to go to the internal static ArgumentException Argument(string messageFormat, params object[] messageArgs) method that is defined here:

https://github.com/aspnet/AspNetWebStack/blob/a55e96a451393aa854556ac9feb2d80e81988a9c/src/Common/Error.cs#L37-L40

Instead, it goes to the internal static ArgumentException Argument(string parameterName, string messageFormat, params object[] messageArgs) method that is defined here:

https://github.com/aspnet/AspNetWebStack/blob/a55e96a451393aa854556ac9feb2d80e81988a9c/src/Common/Error.cs#L49-L52

The format string that was read from Properties.Resources.FormUrlEncodedMismatchingTypes thus becomes misused as the parameter name.

I have not tried to reproduce this bug in practice, but the incorrect call is evident from the IL disassembly of lib/netstandard2.0/System.Net.Http.Formatting.dll in the Microsoft.AspNet.WebApi.Client 6.0.0 package:

  IL_001b:  call       string System.Net.Http.Properties.Resources::get_FormUrlEncodedMismatchingTypes()
  IL_0020:  ldarg.1
  IL_0021:  ldarg.1
  IL_0022:  ldlen
  IL_0023:  conv.i4
  IL_0024:  ldc.i4.1
  IL_0025:  sub
  IL_0026:  call       string System.Net.Http.Formatting.FormUrlEncodedJson::BuildPathString(string[],
                                                                                             int32)
  IL_002b:  call       !!0[] [netstandard]System.Array::Empty<object>()
  IL_0030:  call       class [netstandard]System.ArgumentException System.Web.Http.Error::Argument(string,
                                                                                                   string,
                                                                                                   object[])
  IL_0035:  throw

In the Microsoft.AspNet.WebApi.Client 4.0.20505 package (released on 31 May 2012), the FormUrlEncodedJson.AddToArray method did not have this bug yet. In the Microsoft.AspNet.WebApi.Client 4.0.20710 package (released on 11 August 2012), the method had the bug. I think the bug was introduced by commit f19f4683cc36ebb2666fa36f6dcd1825f4b214aa in May 2012.

Because the bug is so old and is not known to cause any problems in practice, I suspect you might decide not to fix it.