assign null value to string in C# and insert SQL Server decimal column that accepts null values

OK, I was afraid you were doing string concatenation. You certainly can continue with this approach, but it's a bit easier to read and work with (not to mention safer) if you use parameterized queries. Try this:

SqlConnection conn = new SqlConnection(connectionString);
string mySQLstring = "INSERT INTO [Table1] (Col1,Col2) VALUES (@col1Param, @col2Param)";

try
{
    SqlCommand cmd = new SqlCommand(mySQLstring,conn);
    
    cmd.Parameters.AddWithValue("@col1Param", DBNull.Value);
    cmd.Parameters.AddWithValue("@col2Param", 0.00);

    cmd.ExecuteNonQuery();
}
catch (Exception ex)
{
    streamwriter to capture exception here
}

conn.Close()

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One benefit of this approach is that you can let the framework worry about how to format strings and numbers to fit the query. You will notice that I don't have any quotes around either parameter within the query. If you were to pass a number (in the AddWithValue call), it will be filled in properly; likewise, if you were to pass a string, then it would be quoted properly when it arrives at the DB.

Note:  I put in some placeholder data above to demonstrate using DBNull.Value. Change these accordingly. If you want to send a numeric value, and that value starts off as string, then you will need to cast it to a numeric type before assigning it to the parameter.

e.g.

...
    cmd.Parameters.AddWithValue("@col2Param", Convert.ToInt32(stringTEST2));
...

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