How do i use linq to do this?

foreach (DataRow row in dtcols.Rows)
{
    if (!avlcols.Exists(x => x.Col开发者_StackOverflow社区Name.ToLower() == row["field_name"].ToString().ToLower()))
    {
        avlcols.Add(new Column() 
            {
                ColName=row["field_name"].ToString(),
                ColWidth=row["field_width"].ToString() 
            });
    }
}

Adding only columns that dont exist in avlcols.

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Change your if statment to

!avlcols.Contains(x => x.ColName.ToLower() == row["field_name"].ToString().ToLower()))

May I also Suggest using syntax like this instead

!avlcols.Contains(x => String.Equals(x.ColName, row["field_name"].ToString(), OrdinalIgnoreCase))

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Replace avlCols.Exists with avlCols.Contains and you're there.

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var rows = from row in dtcols.Cast<DataRow>()
           where !avlcols.Exists(x => x.ColName.Equals(row["field_name"].ToString(),StringComparison.OrdinalIgnoreCase))
           select row;
foreach(var row in rows)
{
    avlcols.Add(new Column()
    {
        ColName = row["field_name"].ToString(),
        ColWidth = row["field_width"].ToString()
    });
}