String manipulation with & or + in VB.NET

Question

I have seen several programmers use & and + for string manipulation.

Such as:

dim firstvar as string
dim secondvar as string
dim thirdvar as string

thirdvar = firstvar & secondvar

Or is it:

thirdvar = firstvar + secondvar

Does it matter? If so, why?

Answer 1

The + and & operators are not identical in VB.NET.

Using the & operator indicates your intention to concatenate strings, while the + operator indicates your intention to add numbers. Using the & operator will convert both sides of the operation into strings. When you have mixed types (one side of the expression is a string, the other is a number), your usage of the operator will determine the result.

1 + "2" = 3 'This will cause a compiler error if Option Strict is on' 1 & "2" = "12" 1 & 2 = "12" "text" + 2 'Throws an InvalidCastException since "text" cannot be converted to a Double' 

So, my guideline (aside from avoiding mixing types like that) is to use the & when concatenating strings, just to make sure your intentions are clear to the compiler, and avoid impossible-to-find bugs involving using the + operator to concatenate.

Answer 2

Consider if you are better off using String.Format when concatenating strings. Usually, the code ends up making more sense that way.

Also, if you concatenate many times, consider using a StringBuilder rather than a String.

Answer 3

In general, & will always concatenate strings regardless of types of arguments, while + will not. Since there's no drawback in using & over + otherwise, in situations where you clearly desire string concatenations, it is preferable. It also makes the intent of code slightly clearer.