C＃字符串查询为SQL Server参数放空

Question

I know this question was asked many times, but i am not able to find a solution for it

This is my code

  string query = @"SELECT *
    FROM SMSMessage
    WHERE (respondCode  IS @respondCode)
and (sentOn > '08/26/2016')
       ";
                //string query = "select * from SMSMessage";
                SqlConnection con = new SqlConnection(Utilities.getConnectionString());
                SqlCommand cmd = new SqlCommand(query, con);
                cmd.Parameters.AddWithValue("@respondCode", DBNull.Value);

I want the responseCode to be null,

I am getting error:

syntax error near @responseCode

when I do this responseCode is NULL , there is no syntax error, but the query for some reaonse doesn't bring any result

Help please

Answer 1

I guess you want this

Where (respondCode = @respondCode or @respondCode is null)
  and sentOn > '08/26/2016'

When a value is passed to @respondCode parameter the records will be filter based on @respondCode and sentOn > '08/26/2016' .

When nothing is passed to @respondCode parameter (ie) NULL , then records will be filtered only based on sentOn > '08/26/2016'

As mentioned in comments by Steve, If you need records only when respondCode is NULL then no need of that variable just hardcode the NULL condition in Where clause

Where respondCode is null
  and sentOn > '08/26/2016'

Answer 2

I would use directly IS NULL and not passing any parameter, but the most important change is how do you apply the date constant in your query statement.
Assuming you use the Italian locale in your sql server database I would use

string query = @"SELECT * SMSMessage
                 WHERE respondCode  IS NULL
                 AND (sentOn > CONVERT(DateTime, '26/08/2016', 105))

T-SQL Convert docs

On the contrary I would look carefully to the value passed for the sentOn condition. If this value changes dynamically it is better to use a parameter for this value. In this way the query optimizer of sql server will be able to build a better (faster) execution plan

string query = @"SELECT * SMSMessage
                 WHERE respondCode  IS NULL
                 AND sentOn > @dateLimit";

SqlConnection con = new SqlConnection(Utilities.getConnectionString());
SqlCommand cmd = new SqlCommand(query, con);
cmd.Parameters.Add("@dateLimit", SqlDbType.DateTime).Value = new DateTime(2016, 8, 26);