DBNull if statement | 易学教程

问题

I'm trying to execute a stored procedure and then use an if statement to check for null values and I'm coming up short. I'm a VB guy so please bear with me if I'm making a schoolboy syntax error.

objConn = new SqlConnection(strConnection);
objConn.Open();
objCmd = new SqlCommand(strSQL, objConn);
rsData = objCmd.ExecuteReader();
rsData.Read();

if (!(rsData["usr.ursrdaystime"].Equals(System.DBNull.Value)))
        {
            strLevel = rsData["usr.ursrdaystime"].ToString();

        }

Would this allow me to check whether the SQL connection is returning just a value and if so then populating my string?

I'm used to being able to just check the below to see if a value is being returned and not sure I'm doing it correctly with C#

If Not IsDBNull(rsData("usr.ursrdaystime"))

Any help would be appreciated!

回答1:

This should work.

if (rsData["usr.ursrdaystime"] != System.DBNull.Value))
{
    strLevel = rsData["usr.ursrdaystime"].ToString();
}

also need to add using statement, like bellow:

using (var objConn = new SqlConnection(strConnection))
     {
        objConn.Open();
        using (var objCmd = new SqlCommand(strSQL, objConn))
        {
           using (var rsData = objCmd.ExecuteReader())
           {
              while (rsData.Read())
              {
                 if (rsData["usr.ursrdaystime"] != System.DBNull.Value)
                 {
                    strLevel = rsData["usr.ursrdaystime"].ToString();
                 }
              }
           }
        }
     }

this'll automaticly dispose (close) resources outside of block { .. }.

回答2:

The idiomatic way is to say:

if(rsData["usr.ursrdaystime"] != DBNull.Value) {
    strLevel = rsData["usr.ursrdaystime"].ToString();
}

This:

rsData = objCmd.ExecuteReader();
rsData.Read();

Makes it look like you're reading exactly one value. Use IDbCommand.ExecuteScalar instead.

回答3:

The closest equivalent to your VB would be (see this):

Convert.IsDBNull()

But there are a number of ways to do this, and most are linked from here

回答4:

Yes, just a syntax problem. Try this instead:

if (reader["usr.ursrdaystime"] != DBNull.Value)

.Equals() is checking to see if two Object instances are the same.

回答5:

Consider:

if(rsData.Read()) {
  int index = rsData.GetOrdinal("columnName"); // I expect, just "ursrdaystime"
  if(rsData.IsDBNull(index)) {
     // is a null
  } else {
     // access the value via any of the rsData.Get*(index) methods
  }
} else {
  // no row returned
}

Also: you need more using ;p

回答6:

if(!rsData.IsDBNull(rsData.GetOrdinal("usr.ursrdaystime")))
{
  strLevel = rsData.GetString("usr.ursrdaystime"); 
}

http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqldatareader.isdbnull.aspx

http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqldatareader.getordinal.aspx

回答7:

I use String.IsNullorEmpty often. It will work her because when DBNull is set to .ToString it returns empty.

if(!(String.IsNullorEmpty(rsData["usr.ursrdaystime"].toString())){
        strLevel = rsData["usr.ursrdaystime"].toString();
    }

回答8:

Ternary operator should do nicely here: condition ? first_expression : second_expression;

strLevel = !Convert.IsDBNull(rsData["usr.ursrdaystime"]) ? Convert.ToString(rsData["usr.ursrdaystime"]) : null

回答9:

At first use ExecuteScalar

 objConn = new SqlConnection(strConnection);
 objConn.Open();
 objCmd = new SqlCommand(strSQL, objConn);
 object result = cmd.ExecuteScalar();
 if(result == null)
     strLevel = "";
 else 
     strLevel = result.ToString();

来源：https://stackoverflow.com/questions/10431835/dbnull-if-statement

标签

dbnull