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January 26, 2020

C# and performance of string building


Strings are immutable in C#. That means that a string never gets modified by any operation. This implies that, when you call Replace for example the operation is never done in-place. Instead a new string containing the replaced parts is built and returned. It has a huge impact on performance if you manipulate big strings that are assembled from many small parts.


Let's assume you want to generate a document on the fly built depending on many internal variables. This string is returned by your method.

Wrong way of solving the problem

If you choose to create an initial string and concatenate the changes at each step, then your solution will be rather slow.

string s = string.empty;
s = s + "New data ";         // the resulting s has to be newly allocated!
s = s + " some other data."; // new memory allocated here!
return s;

On each step a new string will be created (strings are immutable)

Better solution

Prefer in such case using the StringBuilder. It is much faster when it comes to handling a lot of small appends to a string. The StringBuilder pattern is well known in Java. Generally you need such a pattern each time the string is immutable to compensate the missing in-place modification

var s = new StringBuilder();
s.Append("New data ");
s.Append(" some other data.");
return s.ToString();


That strings are immutable is not a drawback or design flaw. It is a design choice that provides an easier per-value comparison operator and better thread safety. If a string object is passed around, it is guaranteed to always hold the same value. A thread having a look at it later will still see the same value than a few minutes ago. This saves from heavy locking mechanisms at nearly no additional cost because even if the transformation were in-place, it still would have to check for reallocations of memory if the buffer is not big enough and in that case also allocate a new buffer.


The StringBuilder is a handy class that you definitively should use when you build strings from many parts.