Best way to check if a Data Table has a null value in it

Try comparing the value of the column to the DBNull.Value value to filter and manage null values in whatever way you see fit.

foreach(DataRow row in table.Rows)
{
    object value = row["ColumnName"];
    if (value == DBNull.Value)
        // do something
    else
        // do something else
}

More information about the DBNull class

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If you want to check if a null value exists in the table you can use this method:

public static bool HasNull(this DataTable table)
{
    foreach (DataColumn column in table.Columns)
    {
        if (table.Rows.OfType<DataRow>().Any(r => r.IsNull(column)))
            return true;
    }

    return false;
}

which will let you write this:

table.HasNull();