如何在C＃中使用另一种Form（或另一类的一个类）中的方法

Question

So, I have a treeview in Form2. On a button click, in Form3 a code executes which inserts the text from the textboxes(from Form3) into the database, after that happens, I want the treeview in Form2 to update itself with those values from the database, which means I need to either use the treeView1(from Form2) in the code I write in Form3 or write a method in Form2(which I've done) to use in Form3.

Method in Form2:

    public static void LoadTree()
    {
        int j = 1;
        string var;
        con.Open();
        OleDbCommand populate = new OleDbCommand("SELECT Project_name FROM Edit_Nodes ORDER BY Location ASC", con);
        OleDbDataReader reader = populate.ExecuteReader();
        while (reader.Read())
        {
            var = "H" + j + " - " + reader["Project_name"] + "";
            treeView1.Nodes[j].Text = var;
            j++;
        }
        con.Close();
    }

Problem : "Error 5 An object reference is required for the non-static field, method, or property 'Tool.Form2.con'" and so on for treeView1 and the other con's in the code.

Code in Form3:

   private void button1_Click(object sender, EventArgs e)
    {
       // more code here

      Form2.LoadTree();
    }

My question is how to solve these errors, OR....how do I directly make the program recognise treeView1 in Form3 (it belonging to Form2) so I can write the code again there.

Answer 1

First, make it an instance method instead of static:

public void LoadTree()
{
    // implementation
}

The reason static doesn't work is because it doesn't have the context of any given instance of the form, so it doesn't have the context of the controls on any given instance (such as treeView1 ). Next, you'll need the Form3 instance to have a reference to that particular Form2 instance. An easy way to do this is to add a property to Form3 and require a reference in its constructor. So, in Form3 :

private Form2 Form2Instance { get; set; }

public Form3(Form2 form2Instance)
{
    this.Form2Instance = form2Instance;
}

At this point any time you need to create an instance of Form3 , you need to supply it an instance of Form2 . So if you create it from within Form2 then you'd do something like this:

var form3Instance = new Form3(this);
form3Instance.Show();

At that point any instance of Form3 has a reference to the instance of Form2 which created it. So your code on Form3 can just reference the instance it has in that private property to call the method:

private void button1_Click(object sender, EventArgs e)
{
    // more code here
    this.Form2Instance.LoadTree();
}

Answer 2

There're many issues with your code, see my comments

  // Since you want to update treeView1 you should 
  // point out on which instance of Form1 the target treeView1 is.
  // That's why LoadTree() can't be static
  public void LoadTree() { // <- Not static!
    // Much better to use a local connection here
    //TODO: provide the right connection string here
    using(OleDbConnection con = new OleDbConnection(connectionString)) {
      con.Open();

      // wrap all local IDisposable into using(...) {...}
      using (OleDbCommand populate = new OleDbCommand(con)) {
        // Format out your SQL for it to be readble
        populate.CommandText = 
          "  select Project_Name\n" + 
          "    from Edit_Nodes\n" +
          "order by Location asc";

        // wrap all local IDisposable into using(...) {...}
        using (OleDbDataReader reader = populate.ExecuteReader()) {
          // When updating visual controls prevent them from constant re-painting
          // and annoying blinking
          treeView1.BeginUpdate();

          try {  
            // What is "j"? No-one knows; "loop" is more readable
            int loop = 1;

            while(reader.Read()) {
              // "var" is unreadable, "nodeText" is clear
              String nodeText = "H" + loop.ToString() + " - " + reader["Project_Name"];

              // Check ranges: what if SQL returns more recods than treeView1's nodes? 
              if (loop >= treeView1.Nodes.Count) 
                break;

              treeView1.Nodes[loop].Text = nodeText;

              loop += 1;
            }
          finally {
            treeView1.EndUpdate();
          }  
        }  
      }
    }

Now, it's time to call LoadTree() ; but for which Form2 instance? Imagine, that you've opened five Form2 's. Now let's call LoadTree() for all Form2 instances:

private void button1_Click(object sender, EventArgs e) {
  // Enumerating all open forms
  foreach (Form form in Application.OpenForms) {
    Form2 source = form as Form2;

    // If the open form is Form2, call LoadTree
    if (!Object.ReferenceEquals(null, source))
      source.LoadTree();
  }
}

Answer 3

It looks like your operating on something called 'treeView1' in your LoadTree() method. If this is part of the Form2 it will have to be static or you will have to call this method from an already instantiated object of it.

1. Make treeView1 static

2. Form2 form = new Form2(); form.TreeView();