Using variables inside strings

In C# 6 you can use string interpolation:

string name = "John";
string result = $"Hello {name}";

The syntax highlighting for this in Visual Studio makes it highly readable and all of the tokens are checked.

Use the following methods

1: Method one

var count = 123;
var message = $"Rows count is: {count}";

2: Method two

var count = 123;
var message = "Rows count is:" + count;

3: Method three

var count = 123;
var message = string.Format("Rows count is:{0}", count);

4: Method four

var count = 123;
var message = @"Rows
                count
                is:{0}" + count;

5: Method five

var count = 123;
var message = $@"Rows 
                 count 
                 is: {count}";

This functionality is not built-in to C# 5 or below.
Update: C# 6 now supports string interpolation, see newer answers.

The recommended way to do this would be with String.Format:

string name = "Scott";
string output = String.Format("Hello {0}", name);

However, I wrote a small open-source library called SmartFormat that extends String.Format so that it can use named placeholders (via reflection). So, you could do:

string name = "Scott";
string output = Smart.Format("Hello {name}", new{name}); // Results in "Hello Scott".

Hope you like it!