string interpolation simple cases are not optimized (such as: string s1 = $"hello")

[dzeitlin]

string s1 = $"hello"; emits string s1= string.Format("hello", new object[0]);
instead of simple: s1 = "hello";

also consider to optimize cases such as:
$"hello" + $" " + $"world" to constant expression evaluation (or at least one string.Format() call, not 3 in this case)

[controlflow]

All this already implemented in master branch: http://tryroslyn.azurewebsites.net/#b:master/K4Zwlgdg5gBAygTxAFwKYFsDcAobBjAGwEMQQYBhGAb2xjphQCdJY4BGGAXgD4YASAEQALVAQIB7ATnoNkzaPABMXXoJFjJMANT8BMPTsEB3cYwIATKdgC+QAAA=

[dzeitlin]

Thanks you right, I chcked this in oficial CTP5 release.
But still even in master branch:
Code like: $"hello {null} {objNull}" is not optimized as a classic string, see:
http://tryroslyn.azurewebsites.net/#b:master/K4Zwlgdg5gBAygTxAFwKYFsDcAobBjAGwEMQQYBhGAb2xjpgAcAnMANyLRjwHsIUZuAIwBWqPMgEiAcsAIEYAXhgRZBHPRi16KFtHgBGRQD4YAEgBEAC1Rzu1FXIC+1IcJlPz67cl2w4AJmMYKxsCbnMYAGplVSjJN1UcRyA

[paulomorgado]

What if the current culture has a formater that does something with nulls?

[stephentoub]

And how common is it in performance-critical situations to be doing string formatting with constant null values? Even if it could be optimized, seems extremely corner-case, or am I missing something?

[mikedn]

Is there a valid reason to ever write something like $"hello"? If not then maybe this should be a compiler warning.

[s-aida]

Perhaps when an interpolated variable refers to null?

var text = $"hello{world}";

[AdamSpeight2008]

Why was String.Format chosen to be the target method? Rather than String.Join?

Dim text = $"hello{world}"

into

Dim text = String.Join(String.Empty,{ "hello",world } )

If any of the arg holes have alignment or format args then make the target String.Format.

[paulomorgado]

Like I said before on this issue, my current culture might have custom formatting for that.

The predictable way is to always be a call string.Format. There's already a pretty good syntax for string.Concat: +. Which is a lot better than string.Join because it doesn't involve an array invocation and iteration just to have an empty string as a separator.

[gafter]

I cannot reproduce this "bug". I see the code in the compiler to perform this optimization and your test appears to show it working properly:

http://tryroslyn.azurewebsites.net/#b:master/K4Zwlgdg5gBAygTxAFwKYFsDcAobBjAGwEMQQYBhGAb2xjpgAcAnMANyLRjwHsIUZuAIwBWqPMgEiAcsAIEYAXhgRZBHPRi16KFtHgBGRQD4YAEgBEAC1Rzu5nAF8gAA