WPF

namespace MyFirstSLApp
{
    public delegate void ControlChangeHandler(object control, int ctrlId);

    public class BaseUserControl : UserControl
    {
        public ControlChangeHandler OnContentChange;
    }
}

<control:BaseUserControl xmlns:dataInput="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Data.Input"  x:Class="MyFirstSLApp.Login"
    xmlns="http://schemas.microsoft.com/client/2007" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlnsBig Grin="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    xmlns:control="clr-namespace:MyFirstSLApp"
    Width="400" Height="200" >
    <Grid x:Name="LayoutRoot" Background="White" Width="400">
        <Grid x:Name="LoginLayout" >
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="70"></ColumnDefinition>
                <ColumnDefinition Width="*"></ColumnDefinition>
                <ColumnDefinition Width="*"></ColumnDefinition>
                <ColumnDefinition Width="*"></ColumnDefinition>
            </Grid.ColumnDefinitions> 
            <Grid.RowDefinitions>
                <RowDefinition Height="20"></RowDefinition>
                <RowDefinition Height="20"></RowDefinition>
                <RowDefinition Height="20"></RowDefinition>
                <RowDefinition Height="*"></RowDefinition>
            </Grid.RowDefinitions>
            <TextBlock Text="User Id:" Grid.Column="0" Grid.Row="0" ></TextBlock>
            <TextBox x:Name="txtUserId" Grid.Column="1" Grid.Row="0" ></TextBox>
            <TextBlock Text="Password:" Grid.Column="0" Grid.Row="1"></TextBlock>
            <PasswordBox x:Name="txtPassword" Grid.Column="1" Grid.Row="1"></PasswordBox>
            <Button x:Name="btnLogin" Content="Login" Click="btnLogin_Click" Grid.Column="0" Grid.Row="2"></Button>
            <!--<Button x:Name="btn" Content="Login" Grid.Column="3" Grid.Row="0"
                    Grid.ColumnSpan="2" Grid.RowSpan="2"></Button>-->
            <TextBlock x:Name="txtError" Text="" Grid.Column="0" Grid.Row="4" Grid.ColumnSpan="4"></TextBlock>
        </Grid>

        <dataInput:ValidationSummary >
        </dataInput:ValidationSummary>
    </Grid>
    
</control:BaseUserControl>

//Notify the subscriber for UserControl change.
private void btnLogin_Click(object sender, RoutedEventArgs e)
{
    if (OnContentChange != null)
            OnContentChange(this, 1);
}

[assembly: XmlnsDefinition("http://schemas.microsoft.com/client/2007", "MyFirstSLApp")]

<UserControl x:Class="MyFirstSLApp.Page"
    xmlns="http://schemas.microsoft.com/client/2007" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:ctrl="clr-namespace:MyFirstSLApp"
    Width="400" Height="300">
    <Grid x:Name="LayoutRoot" Background="White">
    </Grid>
</UserControl>

private void Application_Startup(object sender, StartupEventArgs e)
{
    //create instance of Container control
    Page page = new Page();
    //Create instance of Login control
    Login login = new Login();
    //Subscribe to the delegate;
    ContentChange(login);
    //Add the Login control to the <Grid>
    page.LayoutRoot.Children.Add(login);
    //Set the container control as main application UI
    this.RootVisual = page;
}

public void ContentChange(BaseUserControl control)
{
    control.OnContentChange += new ControlChangeHandler(control_OnContentChange);
}

void control_OnContentChange(object control, int ctrlId)
{
    Page page = null;
    switch (ctrlId)
    { 
        case 1:
            //Retrieve the container control
            page = (Page)this.RootVisual;
            //Clear all the element from Children collection of the <Grid>
            page.LayoutRoot.Children.Clear();
            //Create instance of Report control and add it to the children collection of <Grid>
            Reports report = new Reports();
            page.LayoutRoot.Children.Add(report);
            //Subscribe to the delegate
            ContentChange(report);
            break;
        case 2:
            page = (Page)this.RootVisual;
            page.LayoutRoot.Children.Clear();
            Login login = new Login();
            page.LayoutRoot.Children.Add(login);
            ContentChange(login);
            break;
    }
}