btnExample.IsEnabled Assignment Throws Exception in WPF

Question

This is my first attempt at using WPF for building a C# application; I have always used Windows Forms in the past. I am running into an error with what seems to be a simple assignment. When the following assignment of a button's .IsEnabled property is attempted, the exception System.Reflection.TargetIncovationException occurs in PresentationFramework.dll. When I press Break, I just get the Source Not Available window and the option ti view disassembly information.

private void txtFileLoc_TextChanged(object sender, TextChangedEventArgs e) {
    if (txtFileLoc.Text.EndsWith(".txt"))
        btnExecute.IsEnabled = true;
    else 
        btnExecute.IsEnabled = false;
}

I have verified that it is the assignment throwing the exception by replacing them with Console.WriteLine to debug.

EDIT: As requested, here is the program XAML & CS

<Window x:Name="winMain" x:Class="IP_Extractor_2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="IP Extractor" Height="250" Width="410" ResizeMode="CanMinimize" Background="#FFECEFF4">
    <Grid x:Name="grdMain">
        <Grid.RowDefinitions>
            <RowDefinition Height="39*"/>
            <RowDefinition Height="161*"/>
            <RowDefinition Height="21*"/>
        </Grid.RowDefinitions>
        <ProgressBar x:Name="prgProgress" Margin="0,0,0,0" Grid.Row="2" VerticalAlignment="Bottom" Height="17" Foreground="#FF7A6BA6" Value="50"/>
        <TextBox x:Name="txtFileLoc" HorizontalAlignment="Left" Height="20" Margin="10,10,0,0" Text="Browse for a Text File" VerticalAlignment="Top" Width="290" TextChanged="txtFileLoc_TextChanged"/>
        <Button x:Name="btnBrowse" Content="Browse" HorizontalAlignment="Left" Margin="305,10,0,0" VerticalAlignment="Top" Width="52" Height="20" Click="btnBrowse_Click"/>
        <Button x:Name="btnExecute" Content="Go" HorizontalAlignment="Left" Margin="362,10,0,0" VerticalAlignment="Top" Width="22" Height="20" Click="btnExecute_Click" IsEnabled="False"/>
        <ListBox x:Name="lboResults" HorizontalAlignment="Left" Height="151" Margin="10,0,0,0" Grid.Row="1" VerticalAlignment="Top" Width="374"/>
    </Grid>
</Window>

CODE:

using Microsoft.Win32;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace IP_Extractor_2
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        OpenFileDialog openFileDialog1;

        public MainWindow() {
            InitializeComponent();
        }

        private void btnBrowse_Click(object sender, RoutedEventArgs e) {
            // Create an instance of the open file dialog box.
            openFileDialog1 = new OpenFileDialog();

            // Set filter options and filter index.
            openFileDialog1.Filter = "Text Files (.txt)|*.txt";
            openFileDialog1.FilterIndex = 1;
            openFileDialog1.Multiselect = false;

            // Call the ShowDialog method to show the dialog box.
            bool? userClickedOK = openFileDialog1.ShowDialog();

            // Process input if the user clicked OK.
            if (userClickedOK == true)
                txtFileLoc.Text = openFileDialog1.FileName;
        }

        private void txtFileLoc_TextChanged(object sender, TextChangedEventArgs e) {
            if (txtFileLoc.Text.EndsWith(".txt"))
                btnExecute.IsEnabled = true;
            else
                btnExecute.IsEnabled = false;
        }
    }
}

Answer 1

Check for the null/empty string before setting and see if that changes things.

if (btnExecute != null)
{
    btnExecute.IsEnabled = ((txtFileLoc != null) &&
                            (string.IsNullOrEmpty(txtFileLoc.Text) == false) &&
                            (txtFileLoc.Text.ToLower().EndsWith(".txt")));
}

Edit

Although code behind works and could be a valid way of doing it, it is not the standard way. WPF is all about binding and INotifyProperty changed events related.

I would have my button bound to a View Model Boolean which would be on the page's data context (which would be inherited by the button) and bound such as

<Button IsEnabled="{Binding IsExecuteButtonOperational }"/>

I recommend you begin to look into MVVM programming paradigm (Think of it as way of separating concerns of business logic form view logic). I provide an example on my blog Xaml: ViewModel Main Page Instantiation and Loading Strategy for Easier Binding.which could get you started.