Sorting Issue in WPF Data Grid

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C#

WPF

I need to create a grid with a list of objects as the datasource -- very basic. I also need to add 2 columns that are always fixed. So when the user sorts on one of the columns from the list of objects these 2 columns never sort. Similar to a Sort in Excel after selecting the sort range.

Example:

A B C D
L 1 16.1 12/2/2012
L 2 17.4 3/14/2011
R 3 8.2 5/15/2012
R 4 9.0 10/19/2012

Not sure how well the above grid will show up, but I need columns A and B to always remain in that order while the user sorts columns C and D. Columns C and D will sort together