Concatenating

To concatenate string variables, you can use the + or += operators, string interpolation or the String.Format, String.Concat, String.Join or StringBuilder.Append methods.

string fullName = firstName + " " + lastName; 
// + operator
string fullName = string.Format("{0} {1}", firstName, lastName); 
// .Format method
string fullName = $"My name is {firstName} and my last name is {lastName}."; 
// Interpolation

String Concat

string[] words = { "The", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog." };
string unreadablePhrase = string.Concat(words);
Console.WriteLine(unreadablePhrase); 
//The output is " Thequickbrownfoxjumpsoverthelazydog. "

String Join

string[] words = { "The", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog." };
string readablePhrase = string.Join(" ", words);
Console.WriteLine(readablePhrase); 
// output: " The quick brown fox jumps over the lazy dog. "

Combine Strings in a Loop

var sb = new System.Text.StringBuilder();
for (int i = 0; i < 20; i++)
{
    sb.AppendLine(i.ToString());
}
Console.WriteLine(sb.ToString());