String Performance: Concatenating Strings

There are several ways to concatenate strings together in .NET. I would say most developers would do it like this.

string result = "David" + "McCarter";

You can also use string.Concat() like this:

string result = string.Concat("David", "McCarter");

You can also use string.Join() like this:

string result = string.Join(string.Empty, "David", "McCarter");

Combining Many Strings

Before we talk about the rest of this section, I must stop and remind you that strings in .NET are immutable. Because of this, combining multiple strings together could cause a performance and memory issue. I see developers doing this in their code often, even for building large SQL queries (which could also be a security issue). 

To benchmark the performance, I am looping through a string array to concatenate the strings. Here is an example.

var result = string.Empty;

for (int count = 0; count < stringArray.Length; count++)
{
    result += stringArray[count];
}

Benchmark Results

Let’s look at the performance of these three methods of combining strings from a string array with different word counts. As you might suspect, using += to concatenate strings for this benchmark allocates a lot more memory than the other two methods.

Allocations

There is a very big difference with the allocations for these three benchmark tests.

• += is 1,848 to 15,039,946 bytes
• Concat() is 400 to 30,096 bytes
• Join() is 400 to 3096 bytes

My recommendation is to use Concat() since it is more performant if you don’t need to separate the strings with a character. If you do, then use string.Join(). I’ve been warning software engineers for a very long time about the performance and memory issues when using +=!

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The benchmark results in this article are only for your reference. Your results will be different, so make sure to benchmark your code using Benchmarkdotnet.