Self explanatory snippet

SqlConnection db = new SqlConnection(Properties.Settings.Default.ConnectionString);

            
try
{
   db.Open();
   SqlDataAdapter dataAdapter = new SqlDataAdapter("SELECT FirstName, LastName  FROM Contacts", db);

   DataTable dataTable = new DataTable();
   dataAdapter.Fill(dataTable);

   foreach(DataRow row in dataTable.Rows)
   {
      string firstName = row["FirstName"];
      string lastName  = row["LastName"];
   }
 
   db.Close(); 
   db.Dispose(); 
   db = null;0
}
catch (Exception e)
{
   System.Windows.Forms.MessageBox.Show(e.Message, "DB error", System.Windows.Forms.MessageBoxButtons.OK, System.Windows.Forms.MessageBoxIcon.Error);      
}

To show a metafile

metaFile = new Metafile(path);
e.Graphics.DrawImage(metaFile, clientRect);    
metaFile.Dispose(); 
metaFile = null;

Although this code shows the metafile, you may notice strange scaling problems. The metafile is being drawn ignoring the aspect ratio!

You can get the original size using the following code

MetafileHeader header = metaFile.GetMetafileHeader();
Rect metaRect = header.Bounds;

Use the metaRect to adjust the clientRect to get the same aspect ratio and the scaling will be fine.

To get the path where the executable is located use the following code:

Path.GetDirectoryName(Application.ExecutablePath);

The returned path does not contain a trailing backslash.

To add a filename or mask use Path.Combine

By default the arrow keys are processed by the form, to use them yourself override the following method.

 protected override bool IsInputKey(Keys keyData)
        {
            switch (keyData)
            {
                case Keys.Up:
                case Keys.Down:
                case Keys.Left:
                case Keys.Right:
                    return (true);
             }
            return base.IsInputKey(keyData);
        }

This error is shown when an attempt to assign a value to a property inside a struct.

e.g

System.Drawing.Point p = new Point();
p.X = 1; //legal of course

Listlist = new List();
list.Add(p);
list[0].X = 123; //Cannot modify the return value of System.Collections.Generic.List.this[int]‘ because it is not a variable

Remember structs are value types. They are processed like integers and only assigned as a whole.
Therefore when accessed from a list a copy of the struct is returned. Assigning to the copy is pointless, hence the error.

You can assign one member by using the following method

System.Drawing.Point p = list[0];
p.X = 123;
list[0] = p;

When using multiple threads controls on a form cannot be called directly. Doing so will throw an exception like ‘Cross-thread operation not valid: Control ‘dc’ accessed from a thread other than the thread it was created on.’

Fortunately there is a rather simple solution for this problem. Use the Invoke method to execute code in the GUI thread.

The invokemethod has the following prototype

public void Invoke(System.Delegate delegate);

For example: from a worker thread a button on a form (Form1)  must be pressed. From inside the thread use the following construct to perform it thread safe.

Invoke((MethodInvoker)delegate
{
   Button1.Click();
});

Invoke is a member of the Control class. To make it work use a control created in the same thread.  In the example above you can use Form1.Invoke() or Button1.Invoke() since they both are created in the same thread.

MethodInvoker is a default delegate definition. Its definition is  delegate void MethodInvoker;

External delegates:

Of course the delegate can be external as well, sometimes this is favorable to get cleaner code or if the delegate is frequently used.

delegate void ClickTheButton();
void clickTheButton()
{
   ...
}

Inside the method

ClickTheButton clickTheButtonDelegate = clickTheButton;
Invoke(clickTheButtonDelegate);

Delegates with parameters

Delegates can also be called with parameters . This uses the second form of Invoke()

public void Invoke(System.Delegate delegate, object [] args);

void clickTheButton(int value)
{
   ...
}

delegate void ClickTheButton(int);

Inside the method

 ClickTheButton clickTheButtonDelegate = clickTheButton;
 Invoke(clickTheButtonDelegate, new object[] {1});