PowerShell formatting for a string

I have a string that I want to insert dynamically a variable. Example;

$tag = '{"number" = "5", "application" = "test","color" = "blue", "class" = "Java"}'

I want to accomplish:

$mynumber= 2
$tag = '{"number" = "$($mynumber)", "application" = "test","color" = "blue", "class" = "Java"}'

I want to have the variable inserted on the string, but it is not going through. I guess the '' sets all as a string. Any recommendations on how should I approach this?

PowerShell test and trial and error. Also Google.

Solution

To add to Mathias' helpful answer:

Mistakenly expecting string interpolation inside '...' strings (as opposed to inside "...") has come up many times before, and questions such as yours are often closed as a duplicate of this post.
However, your question is worth answering separately, because:
- Your use case introduces a follow-up problem, namely that embedded " characters cannot be used as-is inside "...".
- More generally, the linked post is in the context of argument-passing, where additional rules apply.

Note: Some links below are to the relevant sections of the conceptual about_Quoting_Rules help topic.

In PowerShell:

only "..." strings (double-quoted, called expandable strings) perform string interpolation, i.e. expansion of variable values (e.g. "... $var" and subexpressions (e.g., "... $($var.Prop)")
not '...' strings (single-quoted, called verbatim strings), whose values are used verbatim (literally).

With "...", if the string value itself contains " chars.:

either escape them as `" or ""
- E.g., with `"; note that while use of $(...), the subexpression operator never hurts (e.g. $($mynumber)), it isn't necessary with stand-alone variable references such as $mynumber:
```
$mynumber= 2
$tag = "{`"number`" = `"$mynumber`", `"application`" = `"test`",`"color`" = `"blue`", `"class`" = `"Java`"}"
```
- Similarly, if you want to selectively suppress string interpolation, escape $ as `$
```
# Note the ` before the first $mynumber.
# -> '$mynumber = 2'
$mynumber = 2; "`$mynumber` = $mynumber"
```
- See the conceptual about_Special_Characters help topic for info on escaping and escape sequences.
- If you need to embed ' inside '...', use '', or use a (single-quoted) here-string (see next).
or use a double-quoted here-string instead (@"<newline>...<newline>"@):
- See Mathias' answer, but generally note the strict, multiline syntax of here-strings:
  - Nothing (except whitespace) must follow the opening delimiter on the same line (@" / @')
  - The closing delimiter ("@ / '@) must be at the very start of the line - not even whitespace may come before it.

Related answers:

Overview of PowerShell's expandable strings
Overview of all forms of string literals in PowerShell
When passing strings as command arguments, they are situationally implicitly treated like expandable strings (i.e. as if they were "..."-enclosed); e.g.
Write-Output $HOME\projects - see this answer.

Alternatives to string interpolation:

Situationally, other approaches to constructing a string dynamically can be useful:

Use a (verbatim) template string with placeholders, with -f, the format operator:

$mynumber= 2
# {0} is the placeholder for the first RHS operand ({1} for the 2nd, ...)
'"number" = "{0}", ...' -f $mynumber # -> "number" = "2", ...

Use simple string concatenation with the + operator:

$mynumber= 2
'"number" = "' + $mynumber + '", ...' # -> "number" = "2", ...