Question: Hai 

am having a WPF user control in my WPF Form, when i click my button in my form , i just want to pass some value to the textbox which is in the usercontrol, tell me how to do this.

Answer: 
There are several ways you can do this. The easiest way is to use a String property and implement INotifyPropertyChanged in your UserControl.

To illustrate, you will have your UserControl like so:

```
/// <summary>
/// Interaction logic for TextBoxUsercontrol.xaml
/// </summary>
public partial class TextBoxUsercontrol : UserControl, INotifyPropertyChanged
{
    private string _text;
    public string Text
    {
        get { return _text; }
        set
        {
            _text = value;
            if(PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Text"));
        }
    }

    public TextBoxUsercontrol()
    {
        DataContext = this;
        InitializeComponent();
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

```

Now the TextBox in your UserControl must bind itself to your Text property like so:

```
<TextBox Text="{Binding Text}" />

```

Then in your WPF form, you will have your UserControl declared and a button to handle the click like so:

```
<local:TextBoxUsercontrol x:Name="textBox" />
<Button Click="ButtonBase_OnClick" >Add Text</Button>

```

And finally, in your Click handler:

```
private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
{
    textBox.Text = "Hello!";
}

```

Having shown you the solution, I give your question asking skill a 1 out of 5. You can be a lot more specific with your question and give sample codes snippets as I have done without asking us to download your whole solution from a site that we must wait download it (not to mention most of us are security conscious about downloading unknown files).

Good luck.
Just Create a Dependency property and Bind the Porperty to the UserControl's TextBox. While creating the object itself assign the value to the Usercontrol's dependency property.