String Interpolation with format variable

Question

I can do this:

var log = string.Format("URL: {0}", url); 

or even like this

var format = "URL: {0}"; ... var log = string.Format(format, url); 

I have a format defined somewhere else and use the format variable, not inline string.

In C# 6, this is seems impossible:

var format = $"URL: {url}"; // Error url does not exist ... var url = "http://google.com"; ... var log = $format; // The way to evaluate string interpolation here 

Is there anyway to use string interpolation with variable declared earlier?

C# 6 seems interpolate the string inline during compile time. However consider using this feature for localization, define a format in config or simply having a format const in a class.

Answer 1

No, you can't use string interpolation with something other than a string literal as the compiler creates a "regular" format string even when you use string interpolation.

Because this:

string name = "bar"; string result = $"{name}"; 

is compiled into this:

string name = "bar"; string result = string.Format("{0}", name); 

the string in runtime must be a "regular" format string and not the string interpolation equivalent.

You can use the plain old String.Format instead.