How should I concatenate strings?

Question

Are there differences between these examples? Which should I use in which case?

var str1 = \"abc\" + dynamicString + dynamicString2;

var str2 = String.Form         


        

Answer 1

var str3 = new StringBuilder
    .AppendFormat("abc{0}{1}", dynamicString, dynamicString2).ToString(); 

the code above is the fastest. so use if you want it fast. use anything else if you dont care.