How to write parts of the process memory to a file

In order to write parts of the process memory to a file use the .writemem command.
Syntax is .writemem FileName Address Range

Example:
You want to dump a huge BSTR into a file:
Address of the BSTR: 0x0d900024

Get the size (The DWORD receedig the actual string contains the size):
0:000> dc 0x0d900024 - 4
0d900020 005f7a1c ...


.writemem c:\temp\string_content.txt 0x0d900024 L?005f7a1c

Please note the "?" in the size parameter to avoid build in size checks.