String interpolation ^{C# 6.0} allows strings to be built up using a syntax from inside the string itself rather than joining variables together with + which can reduce the noise in many scenarios.

C# 10.0 finally allows you to use this feature for constant strings as well, provided every placeholder is itself a constant string.

Code

Now

const string Scheme = "https";
const string Host = "api.example.com";
const string BaseUrl = $"{Scheme}://{Host}/v1";

Before

const string Scheme = "https";
const string Host = "api.example.com";
const string BaseUrl = Scheme + "://" + Host + "/v1";

Notes

• Numeric constants are not allowed as placeholders because formatting them depends on the current culture
• nameof produces a constant string so it works too, which is handy in attributes such as [Obsolete($"Use {nameof(SendAsync)} instead")]

More information

• Specification
• C# Reference