What is the difference between a mutable and immut

Question

What is the difference between a mutable and immutable string in C#?

Answer 1

From http://yassershaikh.com/what-is-the-difference-between-strings-and-stringbuilder-in-c-net/

Short Answer : String is immutable – whereas StringBuilder is mutable.

What does that mean ? Wiki says : In object-oriented, an immutable object is an object whose state cannot be modified after it is created. This is in contrast to a mutable object, which can be modified after it is created.

From the StringBuilder Class documentation:

The String object is immutable. Every time you use one of the methods in the System.String class, you create a new string object in memory, which requires a new allocation of space for that new object.

In situations where you need to perform repeated modifications to a string, the overhead associated with creating a new String object can be costly.

The System.Text.StringBuilder class can be used when you want to modify a string without creating a new object. For example, using the StringBuilder class can boost performance when concatenating many strings together in a loop.

Answer 2

The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.

Answer 3

in implementation detail.

CLR2's System.String is mutable. StringBuilder.Append calling String.AppendInplace (private method)

CLR4's System.String is immutable. StringBuilder have Char array with chunking.

Answer 4

String is immutable

i.e. strings cannot be altered. When you alter a string (by adding to it for example), you are actually creating a new string.

But StringBuilder is not immutable (rather, it is mutable)

so if you have to alter a string many times, such as multiple concatenations, then use StringBuilder.

Answer 5

Immutable :

When you do some operation on a object, it creates a new object hence state is not modifiable as in case of string.

Mutable

When you perform some operation on a object, object itself modified no new obect created as in case of StringBuilder

Answer 6

In .NET System.String (aka string) is a immutable object. That means when you create an object you can not change it's value afterwards. You can only recreate a immutable object.

System.Text.StringBuilder is mutable equivalent of System.String and you can chane its value

For Example:

class Program
{
    static void Main(string[] args)
    {

        System.String str = "inital value";
        str = "\nsecond value";
        str = "\nthird value";

        StringBuilder sb = new StringBuilder();
        sb.Append("initial value");
        sb.AppendLine("second value");
        sb.AppendLine("third value");
    }
}

Generates following MSIL : If you investigate the code. You will see that whenever you chane an object of System.String you are actually creating new one. But in System.Text.StringBuilder whenever you change the value of text you dont recreate the object.

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       62 (0x3e)
  .maxstack  2
  .locals init ([0] string str,
           [1] class [mscorlib]System.Text.StringBuilder sb)
  IL_0000:  nop
  IL_0001:  ldstr      "inital value"
  IL_0006:  stloc.0
  IL_0007:  ldstr      "\nsecond value"
  IL_000c:  stloc.0
  IL_000d:  ldstr      "\nthird value"
  IL_0012:  stloc.0
  IL_0013:  newobj     instance void [mscorlib]System.Text.StringBuilder::.ctor()
  IL_0018:  stloc.1
  IL_0019:  ldloc.1
  IL_001a:  ldstr      "initial value"
  IL_001f:  callvirt   instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::Append(string)
  IL_0024:  pop
  IL_0025:  ldloc.1
  IL_0026:  ldstr      "second value"
  IL_002b:  callvirt   instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::AppendLine(string)
  IL_0030:  pop
  IL_0031:  ldloc.1
  IL_0032:  ldstr      "third value"
  IL_0037:  callvirt   instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::AppendLine(string)
  IL_003c:  pop
  IL_003d:  ret
} // end of method Program::Main