[C#] How to fix "object reference not set to an instance of an object"

Oh! No...
When you coding with .Net. Many time you meet an error: "object reference not set to an instance of an object".
What happen???

Example 1: Convert null to int

string query = "Select * from USERS where ID = '" + txtUID.Text + "'";
SqlCommand cmd = new SqlCommand(query , CN);
int count = (int)cmd.ExecuteScalar();//Error here

What problem? ExecuteScalar return to null when record not found in database. In case, you can using Convert class

Convert.ToInt32(cmd.ExecuteScalar());//It will return 0 when ExecuteScalar() return null

Example 2: Try get object properties when this object is null

var str= null;
if(str!="")// Error here
{
//do some thing
}

Resolve the problem: You can try

var str= null;
if(str !=null && str!="")// Error here
{
//do some thing
}

Or try this

var str= null;
if(!String.IsNullOrEmpty(str))// Error here
{
//do some thing
}

Example: Very much error!

// About 10000 lines

Resole all with

try{
//All code here
}
catch (NullReferenceException ex)
        {
            ShowMessageBox("Error" + ex.Message);
        }