score:2

Accepted answer
var concatenatedkeys = keys.select(m => m[0] + "~" + m[1]).tolist();

var filtered = 
     datacontext.t.where(s => concatenatedkeys.contains(
                                             s.column0 ?? string.empty + 
                                             "~" + 
                                             s.column1));

score:0

var filtered = 
     datacontext.t.where(s => concatenatedkeys.contains(
                                             s.anotherentity.column0 ?? string.empty + 
                                             "~" + 
                                             s.anotherentity.column1));

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