Refactor: Append interpolated or concatenated string to Append calls

[wmjordan]

Given the following code:

var sb = new StringBuilder();
var var1 = DateTime.Now.ToString();

sb.Append($"ABC{var1}EFG");

The third line used string interpolation, which is unnecessary, can be refactored to a series of Append calls as the following code shows.

sb.Append("ABC").Append(var1).Append("EFG");

Moreover, it is also useful to change Append concatenated string to Append calls as well. For instance, the following can also be refactored to the above statement.

sb.Append("ABC" + var1 + "EFG");

// or

sb.Append(String.Concat("ABC", var1, "EFG"));

[josefpihrt]

Following code will use AppendInterpolatedStringHandler under the hood which is even better then StringBuilder.Append methods.

sb.Append($"ABC{var1}EFG");

SharpLab: https://sharplab.io/#v2:CYLg1APgAgTAjAWAFBQAwAIpwHQBUCmAHgC4Dcyys6AwsgN7LpOYAs6AsgBQCUjzDSZkPQA3AIYAndAGcARugC86AHb4A7ugDKxCQEtlAcwBCAV10AbYPgk9yg4U3FSncRegAiY4vly6AtvjYAHIA9mp4Idp6hrYU9g5y2ACCAA4p+MrAnAAkAERJRtR0LgC+AKIAYgDiudx2QiXIjUhAA==

Following code will be converted into previous case by RCS1267.

sb.Append(String.Concat("ABC", var1, "EFG"));

Following code could be added to RCS1267 as an improvement.

sb.Append("ABC" + var1 + "EFG");

[wmjordan]

Yes, I know about the AppendInterpolatedStringHandler thing.
However, in my practical situation, there are quite a few Append calls around this one. Thus the StringBuilder can not be simply substituted by a interpolated string. For example,

var sb = new StringBuilder();
var var1 = DateTime.Now.ToString();

for (var i = 0; i = 100; i++) {
    sb.Append(somethingElse(i));
}
sb.Append($"ABC{var1}EFG");

or sometimes, the sb.Append(interpolated string) is invoked within a loop. So, it is arguable that the string interpolation will bring ultimate benefit to the overall performance.

Leaving the AppendInterpolatedStringHandler thing there, will generate a lot of intermediate string instances, cause extra memory copying, and put burden to the GC. If we break it into Append calls, there will be no such burden then.

[josefpihrt]

Thus the StringBuilder can not be simply substituted by a interpolated string.

Not sure if I understand. Why it couldn't be substitued?

sb.Append(interpolated string) is invoked within a loop

Why it matters if it's in a loop or not?

will generate a lot of intermediate string instances, cause extra memory copying

Where do you see these allocations in the SharpLab example?

[wmjordan]

You are right. From .NET 6 on, there is no significant performance degrade when calling StringBuilder.Append(interpolated string).

Sorry, I did not mentioned that the situation was that the code was compiled for the old .NET Framework where no AppendInterpolatedStringHandler exists. I checked the decompiled code from an assembly and the code is compiled as a call to String.Format, then StringBuilder.Append(string).