How compiler handles string interpolation [duplicate]

Question

I am using string interpolation for a method attribute like -

const string User = "SomeUser";
const string Admin = "Admin";
.
.
.
[Authorize(Roles = $"{User},{Admin}")]
public IHttpActionResult Get()

But Visual Studio gives an error -

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

When I try "+" for string concatenation it works

[Authorize(Roles = User + "," + Admin)]
public IHttpActionResult Get()

Even if I replace "," with ',' it gives the same error.

I wonder how the compiler is handling string interpolation?

String interpolation is just syntactic sugar for a call to string.Format, which is not a constexpr, and attribute arguments need to be constexprs. There are numerous proposals in the C# language repo regarding changing this behaviour to transform string.Format to a constexpr in certain cases like these, e.g. github.com/dotnet/csharplang/issues/384 — Ian Kemp - SO dead by AI greed
– Ian Kemp - SO dead by AI greed, Commented Feb 18, 2019 at 5:04

Andrew · Accepted Answer · 2019-02-18 05:04:31Z

1

string interpolation is converted to string.Format which isn't a compile time constant as it requires kindly review this question for more information

answered Feb 18, 2019 at 5:04

Andrew

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