将对象转换为字符串有错误

Question

My code is this :

SqlConnection scn = new SqlConnection(ClsPublic.GetConnectionString());
                   SqlCommand scm = new SqlCommand("SELECT Name FROM Table WHERE (Blogger = @Blogger)", scn);
                   scm.Parameters.AddWithValue("@Blogger", lblBloger.Text);
                   scn.Open();
                   MyLabel.Text = scm.ExecuteScalar().ToString();
                   scn.Close();

in this line :

lblLastNo.Text = scm.ExecuteScalar().ToString();

Has this error :

Object reference not set to an instance of an object.

or when I using if statement , shows same error

object Blogger= "";
if (Blogger.ToString() != string.Empty)
{
    ....
}

in below code again shows same error .

Answer 1

Most likely scm.ExecuteScalar() is bringing you a null value. You need to test the value returned before using it:

var result = scm.ExecuteScalar();
MyLabel.Text = result == null ? '' : result.ToString();

Answer 2

scm.ExecuteScalar() is returning null, or scm is null, or lblLastNo is null. That's the only reason you get the error 'Object reference not set ..'.

Answer 3

ExecuteScalar return an Object type. This is why you have the same behavior on scm.ExecuteScalar().ToString(); or Blogger.ToString() .

The object type default implementation of the ToString method returns the fully qualified name of the type of the Object, as the following example shows.

If this object is NULL, you will receive the error Object reference not set to an instance of an object.

Your second case with :

object blogger= "";
if (blogger.ToString() != string.Empty)
{
    ....
}

Should not throw an Exception but return a string that represents the object instance. For example : "YourNameSpace.Blogger"

Answer 4

Please enclose with try catch block, so that you can handle runtime exceptions and its makes life easier to understand the problem. and for if (Blogger.ToString() != string.Empty) this u can check for Null condition or if its string. Then you can check for String.IsNullorEmpty

  string str=blogger.toString();
    if (String.IsNullOrEmpty(str)) 
            //do something
   else 
     //do other part

String Is Null or EMpty