The difference between + and & for joining strings

问题:

What is the difference between + and & for joining strings in VB.NET?

回答1:

There\'s no difference if both operands are strings. However, if one operand is a string, and one is a number, then you run into problems, see the code below.

\"abc\" + \"def\" = \"abcdef\"
\"abc\" & \"def\" = \"abcdef\"
\"111\" + \"222\" = \"111222\"
\"111\" & \"222\" = \"111222\"
\"111\" & 222 = \"111222\"
\"111\" + 222 = 333
\"abc\" + 222 = conversion error

Therefore I recommend to always use & when you mean to concatenate, because you might be trying to concatenate an integer, float, decimal to a string, which will cause an exception, or at best, not do what you probably want it to do.

回答2:

The & operator always makes sure that both operands are strings, while the + operator finds the overload that matches the operands.

The expression 1 & 2 gives the value \"12\", while the expression 1 + 2 gives the value 3.

If both operands are strings, there is no difference in the result.

回答3:

None.

As you can see below. These two lines of code compiles exactly to the same IL code:

Module Module1

Sub Main()
    Dim s1 As String = \"s1\"
    Dim s2 As String = \"s2\"
    s2 += s1
    s1 &= s2
End Sub

End Module

Compiles to (note System.String::Concat):

.method public static void  Main() cil managed
{
.entrypoint
.custom instance void [mscorlib]System.STAThreadAttribute::.ctor() = ( 01 00 00 00 ) 
// Code size       31 (0x1f)
.maxstack  2
.locals init ([0] string s1,
       [1] string s2)
IL_0000:  nop
IL_0001:  ldstr      \"s1\"
IL_0006:  stloc.0
IL_0007:  ldstr      \"s2\"
IL_000c:  stloc.1
IL_000d:  ldloc.1
IL_000e:  ldloc.0
IL_000f:  call       string [mscorlib]System.String::Concat(string,
                                                          string)
IL_0014:  stloc.1
IL_0015:  ldloc.0
IL_0016:  ldloc.1
IL_0017:  call       string [mscorlib]System.String::Concat(string,
                                                          string)
IL_001c:  stloc.0
IL_001d:  nop
IL_001e:  ret
} // end of method Module1::Main

回答4:

The + operator can be either addition or concatenation. The & is only concatenation. If the expressions are both strings the results would be the same.

I use & when working with strings, and + when working with numbers, so there is never confusion about my intent. If you mistakenly use + and one expression is a string and one is a number, you run the risk of un-desired results.

回答5:

There is no difference in most of the cases. However, the best practice is:

\"+\" should be reserved for integer additions, because if you don\'t use Option Strict On then you might have really messed up situations such as:

Input + 12 might give you 20 instead of 812. This can be especially bad in an ASP.NET application where the input comes from POST/GET.

Simply put: For joining strings, always use \"&\" instead of \"+\".

Obviously, use StringBuilder where it\'s suitable :)

回答6:

If both of the types are statically typed to System.String, there is zero difference between the code. Both will resolve down to the String.Concat member (this is what + does for strings).

However, if the objects are not strongly typed to string, Visual Basic late binding will kick in and go two very different routes. The + version will attempt to do an add operation which literally tries to add the objects. This will do all manner of attempts to convert both values to a number and then add them.

The & operator will attempt to concatenate. The Visual Basic runtime will go through all manner of conversions to convert both values to strings. It will then String.Concat the results.

回答7:

Straight from MSDN Documentation: Concatenation Operators in Visual Basic

Differences Between the Two Concatenation Operators

The + Operator (Visual Basic) has the primary purpose of adding two numbers. However, it can also concatenate numeric operands with string operands. The + operator has a complex set of rules that determine whether to add, concatenate, signal a compiler error, or throw a run-time InvalidCastException exception.

The & Operator (Visual Basic) is defined only for String operands, and it always widens its operands to String, regardless of the setting of Option Strict. The & operator is recommended for string concatenation because it is defined exclusively for strings and reduces your chances of generating an unintended conversion.

Do trust MSDN! :-)

回答8:

None when joining strings:

    Dim string1 As String = \"A\" + \"B\"
    Dim string2 As String = \"A\" & \"B\"

    If string1.Equals(string2) And string2.Equals(string1) Then
        Debugger.Break()
    End If