S1643 — Strings should not be concatenated using '+' in a loop

Why is this an issue?

Concatenating multiple string literals or strings using the + operator creates a new string object for each concatenation. This can lead to a large number of intermediate string objects and can be inefficient. The StringBuilder class is more efficient than string concatenation, especially when the operator is repeated over and over as in loops.

How to fix it

Replace string concatenation with StringBuilder.

Code examples

Noncompliant code example

string str = "";
for (int i = 0; i < arrayOfStrings.Length ; ++i)
{
  str = str + arrayOfStrings[i];
}

Compliant solution

StringBuilder bld = new StringBuilder();
for (int i = 0; i < arrayOfStrings.Length; ++i)
{
  bld.Append(arrayOfStrings[i]);
}
string str = bld.ToString();

Resources

Documentation

• Microsoft Learn - StringBuilder Class

Benchmarks

Glossary

• Mean
• Standard Deviation

The results were generated by running the following snippet with BenchmarkDotNet:

[Params(10_000)]
public int Iterations;

[Benchmark]
public void StringConcatenation()
{
    string str = "";
    for (int i = 0; i < Iterations; i++)
    {
        str = str + "append";
    }
}

[Benchmark]
public void StringBuilder()
{
    StringBuilder builder = new StringBuilder();
    for (int i = 0; i < Iterations; i++)
    {
        builder.Append("append");
    }
    _ = builder.ToString();
}

Hardware Configuration:

BenchmarkDotNet v0.14.0, Windows 10 (10.0.19045.5247/22H2/2022Update)
12th Gen Intel Core i7-12800H, 1 CPU, 20 logical and 14 physical cores
  [Host]               : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256
  .NET 9.0             : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2
  .NET Framework 4.8.1 : .NET Framework 4.8.1 (4.8.9282.0), X64 RyuJIT VectorSize=256