How to Send and Receive Messages Between Windows Forms Applications

Environment

Product Version	Product	Author
2022.3.913	UI for WinForms	Desislava Yordanova

Description

This article demonstrates a general approach for transferring messages between two separate Windows Forms Applications.

Solution

It is necessary to create two separate Windows Forms Applications, one with name "SendMessagesApp" and another one called "ReceiveMessagesApp".

SendMessagesApp

ReceiveMessagesApp

We will use the SendMessage function passing WM_SETTEXT with the input available in a RadTextBox control. The FindWindowByCaption function is used for finding the handle of the "receiver" form. Then, the form that is expecting to receive the message from another application, should override its WndProc method and execute the desired action - in our case this will be inserting the text in a RadRichTextEditor.

This is the code added to the SendMessagesApp:

• C#
• VB.NET

private void radTextBox1_TextChanged(object sender, EventArgs e)
{
    string text = this.radTextBox1.Text; 

    parenthWnd = FindWindowByCaption(IntPtr.Zero, lpszParentWindow);
    if (!parenthWnd.Equals(IntPtr.Zero))
    {
        SendMessage(parenthWnd, WM_SETTEXT, (IntPtr)1, text);
    }
}

private const int WM_SETTEXT = 0x000C; 
[DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
static extern IntPtr FindWindowByCaption(IntPtr ZeroOnly, string lpWindowName);

[DllImport("User32.dll")]
public static extern Int32 SendMessage(
    IntPtr hWnd, // handle to destination window
    int Msg, // message
    IntPtr wParam, // first message parameter
    String lParam); // second message parameter

string lpszParentWindow = "ReceiveMessagesForm";
IntPtr parenthWnd = new IntPtr(0);

Private Sub RadTextBox1_TextChanged(sender As Object, e As EventArgs) Handles RadTextBox1.TextChanged
    Dim text As String = Me.RadTextBox1.Text
    parenthWnd = FindWindowByCaption(IntPtr.Zero, lpszParentWindow)

    If Not parenthWnd.Equals(IntPtr.Zero) Then
        SendMessage(parenthWnd, WM_SETTEXT, CType(1, IntPtr), text)
    End If
End Sub

Private lpszParentWindow As String = "ReceiveMessagesForm"
Private parenthWnd As IntPtr = New IntPtr(0)

Private Const WM_SETTEXT As Integer = &HC
<DllImport("user32.dll", EntryPoint:="FindWindow", SetLastError:=True)>
Private Shared Function FindWindowByCaption(ByVal ZeroOnly As IntPtr, ByVal lpWindowName As String) As IntPtr

End Function
<DllImport("User32.dll")>
Public Shared Function SendMessage(ByVal hWnd As IntPtr, ByVal Msg As Integer, ByVal wParam As IntPtr, ByVal lParam As String) As Int32
End Function

This is the code added to the ReceiveMessagesApp:

• C#
• VB.NET

public ReceiveMessagesForm()
{
    InitializeComponent();

    this.radRichTextEditor1.Text = "Once upon a time..."; 
} 

private const int WM_SETTEXT = 0x000C;
protected override void WndProc(ref Message m)
{
    if (m.Msg == WM_SETTEXT)
    {
        if (m.WParam == (IntPtr)1)
        {
             this.radRichTextEditor1.Text= Marshal.PtrToStringAuto(m.LParam);
        }
    }
    else
    {
        base.WndProc(ref m);
    }
}

Public Sub New()
    InitializeComponent()
    Me.RadRichTextEditor1.Text = "Once upon a time..."
End Sub

Private Const WM_SETTEXT As Integer = &HC

Protected Overrides Sub WndProc(ByRef m As Message)
    If m.Msg = WM_SETTEXT Then

        If m.WParam = CType(1, IntPtr) Then
            Me.RadRichTextEditor1.Text = Marshal.PtrToStringAuto(m.LParam)
        End If
    Else
        MyBase.WndProc(m)
    End If
End Sub

It is important to note that the Text of the receiver form is set to "ReceiveMessagesForm" since the handle is found by the caption.