String.Concat

With concat, strings are merged (combined). It is possible to concatenate 2 or more strings with several syntax forms in the C# language.

Syntax notes

We use the plus operator and the string.Concat method. The plus compiles into string.Concat. Another option is string.Join.

An example

We concatenate 2 strings with plus. We then use the string.Concat method. The C# compiler converts the plus operator into string.Concat.

using System;

// ... Create a new string reference.
//     It points to the literal.
string s1 = "string2";

// ... Add another string to the start.
string s2 = "string1" + s1;
Console.WriteLine(s2);
string1string2

Two strings

This example shows combining 2 strings with string.Concat. It is the same as using plus. Usually, calling string.Concat directly results in less clear code.

using System;

string s1 = "string2";
string s2 = string.Concat("string1", s1);
Console.WriteLine(s2);
string1string2

Three strings

Here we use the same string.Concat or plus operator. You can use the + in any order, just like method arguments—which essentially is what the operands become.

using System;

string s1 = "string1";
string s2 = "string2";

// Combine 3 strings.
string s3 = s1 + s2 + "string3";

// Write the result to the screen.
Console.WriteLine(s3);
string1string2string3

List

We create a List instance and add 3 string literals to it. We can pass the List variable reference to the string.Concat method. It will concatenate all the strings with no separator.

using System;
using System.Collections.Generic;

// Create list of 3 strings.
var list = new List<string>();
list.Add("Cat");
list.Add("Dog");
list.Add("Perls");

// Concat the list.
string concat = string.Concat(list);
Console.WriteLine(concat);

// Join the list.
string join = string.Join("", list);
Console.WriteLine(join);
CatDogPerls
CatDogPerls

Benchmark, Concat

Sometimes we can use Concat (or the plus operator) or string.Format. Should we prefer Concat or Format when we have the choice?

using System;
using System.Diagnostics;

const int _max = 1000000;
string value1 = "bird";
string value2 = "frog";
string value3 = "dog";

var s1 = Stopwatch.StartNew();
// Version 1: use string.Concat.
for (int i = 0; i < _max; i++)
{
    string result = value1 + value2 + value3;
    if (result == null)
    {
        return;
    }
}
s1.Stop();
var s2 = Stopwatch.StartNew();
// Version 2: use string.Format.
for (int i = 0; i < _max; i++)
{
    string result = string.Format("{0}{1}{2}", value1, value2, value3);
    if (result == null)
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
 34.33 ns    +
124.46 ns    string.Format

Performance, overloads

In the .NET Framework, there are Concat methods that accept between 2 and 4 parameters. And there is one that accepts an array.

Notes, internals

The internals use ConcatArray, which seems to have a different implementation. This overload does not perform as well as the ones with fewer parameters.

StringBuilder

If you have a loop or could have many more than 5 strings, use StringBuilder. This may perform worse, but it will prevent edge cases from causing problems.

Notes, implementation

Should we use string.Concat or string.Join? The implementations .NET are the same except string.Join repeatedly appends the separator.

Strings are frequently combined (concatenated). The benchmarks here give some data data points about what statements are most efficient.