<Window x:Class="WpfCsSimpleDragDropTextFile.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="Basic Drag and Drop Text File into TextBox" Height="200" Width="450" MinWidth="450" MinHeight="200" WindowStartupLocation="CenterScreen" ResizeMode="CanResizeWithGrip">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="40"/>
        </Grid.RowDefinitions>
        <TextBox x:Name="TextBlockInput" Text="Drop text file here..." Grid.Row="0" IsReadOnly="True" PreviewDragOver="TextBlockInput_PreviewDragOver" DragEnter="TextBlockInput_DragEnter" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto"/>
        <TextBox x:Name="TextBlockFullPath" Text="Full Path" Grid.Row="1" IsReadOnly="True" HorizontalScrollBarVisibility="Visible" Margin="0,0,15,0" BorderThickness="0,0,0,0" />
    </Grid>
</Window>

using System.IO;
using System.Windows;

/* Re-created by AlexHaack (alexhaack.de) */

namespace WpfCsSimpleDragDropTextFile
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            TextBlockInput.AllowDrop = true;
        }

        private void TextBlockInput_PreviewDragOver(object sender, DragEventArgs e)
        {
            try
            {
                if (e.Data.GetDataPresent(DataFormats.FileDrop))
                {
                    string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
                    foreach (string filePath in files)
                    {
                        TextBlockInput.Text = File.ReadAllText(filePath);
                        TextBlockFullPath.Text = filePath.ToString();
                    }
                }
            }
            catch (System.Exception ex)
            {
                MessageBox.Show(ex.Message, "Error", MessageBoxButton.OK, MessageBoxImage.Error);
            }
        }

        private void TextBlockInput_DragEnter(object sender, DragEventArgs e)
        {
            try
            {
                if (e.Data.GetDataPresent(DataFormats.FileDrop))
                {
                    e.Effects = DragDropEffects.Copy;
                }
                else
                {
                    e.Effects = DragDropEffects.None;
                }
            }
            catch (System.Exception ex)
            {
                MessageBox.Show(ex.Message,"Error",MessageBoxButton.OK,MessageBoxImage.Error);
            }
        }
    }
}

using System;
using System.IO;
using System.Linq;
using System.Windows.Forms;

/*
 * Source code from:
 * https://www.c-sharpcorner.com/blogs/drag-and-drop-file-on-windows-forms1
 * 
 */

namespace CsWindowsFormsSimpleDragDropTextFileDemo
{
    public partial class MainForm : Form
    {
        public MainForm()
        {
            InitializeComponent();
        }

        private void TextDragDrop_DragOver(object sender, DragEventArgs e)
        {
            try
            {
                if (e.Data.GetDataPresent(DataFormats.FileDrop))
                    e.Effect = DragDropEffects.Link;
                else
                    e.Effect = DragDropEffects.None;
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error message: " + Environment.NewLine + Environment.NewLine + ex.Message, "An error occurred", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private void TextDragDrop_DragDrop(object sender, DragEventArgs e)
        {
            try
            {
                string[] files = e.Data.GetData(DataFormats.FileDrop) as string[]; // get all files droppeds  
                if (files != null && files.Any())
                textBoxContentDragDrop.Text = File.ReadAllText(files.First()); // Read the content from the file
                textBoxDragDropFilePath.Text = files.First(); //select the first one
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error message: " + Environment.NewLine + Environment.NewLine + ex.Message, "An error occurred", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
    }
}