Question

How do I check if a specified column allows null values or not?

I’m using the following code to print all the columns, but I also want to print if the column allows null values:

cnn = new SqlConnection(connetionString);
cnn.Open();

SqlCommand myCommand = new SqlCommand("select * from " + tableName, cnn);
SqlDataAdapter da = new SqlDataAdapter(myCommand);
DataSet ds = new DataSet();

da.Fill(ds, tableName);

foreach (DataColumn dc in ds.Tables[0].Columns)
{
     // Print stuff here, dc.ColumnName is the column name
}

The DataColumn.allowDBnull property doesn’t seem to work when getting a predefined table, it’s always set to true, even in columns that doesn’t allow nulls.

Thanks for your time!

Answer 1

If you are only after column data I would do this from the system views, rather than relying on the data adapter. e.g.

DECLARE @TableName VARCHAR(50) = 'dbo.TableName'

SELECT  Name, Column_ID, Is_Nullable
FROM    SYS.COLUMNS
WHERE   [Object_ID] = OBJECT_ID(@TableName)

This also means you can use parameterised queries properly and avoid the risks of SQL Injection. So your final code would be something like this:

using (var connection = new SqlConnection(connectionString))
using (var command = new SqlCommand("SELECT Name, Is_Nullable FROM sys.columns WHERE [Object_ID] = OBJECT_ID(@TableName)", connection))
{
    connection.Open();
    command.Parameters.Add("@TableName", SqlDbType.VarChar).Value = tableName;
    using (var reader = command.ExecuteReader())
    {
        while (reader.Read())
        {
            Console.WriteLine("Name: {0}; Allows Null: {1}", reader.GetString(0), reader.GetBoolean(1));
        }
    }
}