DataView dv = dt.DefaultView;
dv.RowFilter = "日期字段 is null or 日期字段  <'" + DateTime.Today.ToString() + "'";
dt = dv.ToTable();

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C# code

DataView dv = dt.DefaultView;
dv.RowFilter = "日期字段 is null or 日期字段  <'" + DateTime.Today.ToString() + "'";
dt = dv.ToTable();