String Performance: Concatenating Strings

Updated January 2023

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

string result = "David" + " " + "McCarter";

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 the code that was used for the report.

var result = string.Empty;

Example #1

string.Concat(this.stringArray);

Example #2

foreach (var item in this._stringArray)
{
    result = string.Concat(result, item, ControlChars.Space);
}

Example #3

foreach (var item in this._stringArray)
{
    result = string.Join(ControlChars.Space, item);
}

Example #4

foreach (string _stringArray in this._stringArray)
{
    result = result + _stringArray + ControlChars.Space;
}

or

foreach (string _stringArray in this._stringArray)
{
    result += _stringArray + ControlChars.Space;
}

The drawback to using Concat() is that adding a separator cannot be used.

Benchmark Results

Let’s look at the performance of these four methods of combining strings from a string array with different word counts.

As you can see, using Concat() is the fastest followed by Join(). Using + or += is close to the same performance except for the array with 250 words to combine.

Allocations

There is a huge difference in the allocations for these three benchmark tests.

• Concat() & Join(): 88 to 7,528 bytes
• + and +=: 144 – 1,010,000 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 += !

You can also use the StringBuilder to improve performance!

String Performance: Combining Strings with the StringBuilder

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