Method not found: 'System.String System.String.Format(System.IFormatProvider, System.String, System.Object)

Question

I have a Web API 2 project with help pages that runs fine locally but throws this error when I push it to Azure:

Method not found: 'System.String System.String.Format (System.IFormatProvider, System.String, System.Object)

I temporarily turned custom errors off so full stack trace can be seen here

The error is originating from this line of code:

string selectExpression = String.Format(CultureInfo.InvariantCulture, MethodExpression, GetMemberName(reflectedActionDescriptor.MethodInfo));

See Line 96 here

The full source code is available on GitHub

I'm not even sure where to go with this one.

Answer 1

According to its MSDN page, the overload you're using is only supported on .NET 4.6.

Either configure the host to run .NET 4.6 or change the target framework of the project to 4.5 and recompile.

In 4.5 there's a params object[] overload which will then be chosen, without having to alter your code.

Answer 2

This doesn't make sense. We've had a line of code like this in our application since 2009

String.Format(CultureInfo.CurrentCulture, "You must specify a new password of {0} or more characters.", _membershipService.MinPasswordLength);

Recently we upped the project to .NET 4.6 and now, for me at least, this line breaks with the same error. So obviously the new overload is breaking something, and the method is not new.

Answer 3

If you can neither upgrade host to 4.6 nor downgrade project to 4.5 there is a workaround : pass an "object[]" as args instead of an "object". So you will force usage of the "params object[]" overload. Example :

return string.Format(formatProvider, "{0:" + format + "}", new object[] { value });

Answer 4

In case this helps anyone. We encountered this issue recently after upgrading our development environment to VS2015 (Our target environment is .Net 4)

Our C++/clr projects had not been setup correctly to use the /clr switch i.e. they were set to no common language support, even though we were using the clr. This didn’t cause an issue until we upgraded to VS2015.

I’m not completely clear on why this works. I’m guessing c++/clr project must bind to a specific version of the CLR runtime at compile time. I’d be interested if someone could explain this more clearly.