"Should be a compile time constant" warning when using nameof

Example: Log.Information($"Hello {nameof(MyClass)}"); causes a “Message template should be a compile time constant” warning, but nameof expressions are evaluated at compile time.

[olsh]

Well, after some research, I think that it’s not a mistake. Yes, the nameof expression is evaluated at compile-time, but the interpolation is not.

The compilation of this code fails

const string a = $"Hello {nameof(String)}";

But this code is valid

const string a = "Hello " + nameof(String);

You may say that this irrelevant in the context of Serilog templates. Because both interpolation and concatenation produce an immutable string, but that’s not completely true either.

Let’s look at IL for the code

Console.WriteLine($"Hello {nameof(String)}");

On .NET Core 3.1+ it creates a constant

IL_0000:  ldstr       "Hello String"
IL_0005:  call        System.Console.WriteLine
IL_000A:  ret       

But on old full .NET Framework string.Format() is called internally

IL_0000:  ldstr       "Hello {0}"
IL_0005:  ldstr       "String"
IL_000A:  call        System.String.Format
IL_000F:  call        System.Console.WriteLine
IL_0014:  ret         

And this is a performance issue https://nblumhardt.com/2014/09/how-not-to-parameterize-serilog-events/

Don’t do this. While never particularly good logging practise (the string concatenation occurs regardless of whether logging is enabled or not, wasting RAM and CPU cycles), with Serilog, this is strongly considered an anti-pattern.

So I’m not sure if we want to treat the interpolation as a constant.

Please let me know what you think.

P.S. There is a proposal for constant interpolated strings. https://github.com/dotnet/csharplang/issues/2951

[olsh]

Well, it works if you target C# 10 C# 10

C# 9