WPFのTextBlockの文字列をマウスで選択、クリップボードにコピーしたい

そのまんまなんですが、既に出来ちゃってるWPFアプリをちょっと改修してTextBlockの文字列コピーできるようにBehaviorをTextBlockの動作を変えます。

Behaviorを使いますから参照にSystem.Windows.Interactivityの追加が必要です。
例によってまずXAMLソースから。

<Window x:Class="WpfApp7TextBlockCutPaste.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"
        xmlns:local="clr-namespace:WpfApp7TextBlockCutPaste"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        mc:Ignorable="d"
        Title="MainWindow" Height="150" Width="300">
    <Window.Resources>
        <ResourceDictionary>
            
        </ResourceDictionary>
    </Window.Resources>
    <Grid HorizontalAlignment="Center" VerticalAlignment="Center">
        <TextBlock Text="テキストです" FontSize="22">
            <i:Interaction.Behaviors>
                <local:TextBlockSelectBehavior/>
            </i:Interaction.Behaviors>
        </TextBlock>
    </Grid>
</Window>

次にBehavior、

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.Documents;
using System.Windows.Input;
using System.Windows.Interactivity;
using System.Windows.Media;

namespace WpfApp7TextBlockCutPaste
{
    public class TextBlockSelectBehavior : Behavior<TextBlock>
    {
        private TextPointer StartSelectPosition;
        private TextPointer EndSelectPosition;
        private String SelectedText = "";

        protected override void OnAttached()
        {
            base.OnAttached();
            AssociatedObject.Initialized += AssociatedObject_Initialized;
            AssociatedObject.Loaded += AssociatedObject_Loaded;
            AssociatedObject.Unloaded += AssociatedObject_Unloaded;
            AssociatedObject.MouseDown += AssociatedObject_MouseDown;
            AssociatedObject.MouseMove += AssociatedObject_MouseMove;
            AssociatedObject.MouseUp += AssociatedObject_MouseUp;
        }

        protected override void OnDetaching()
        {
            AssociatedObject.Initialized -= AssociatedObject_Initialized;
            AssociatedObject.Loaded -= AssociatedObject_Loaded;
            AssociatedObject.Unloaded -= AssociatedObject_Unloaded;
            AssociatedObject.MouseDown -= AssociatedObject_MouseDown;
            AssociatedObject.MouseMove -= AssociatedObject_MouseMove;
            AssociatedObject.MouseUp -= AssociatedObject_MouseUp;
            base.OnDetaching();
        }

        private void AssociatedObject_Initialized(object sender, EventArgs e)
        {
            // マウスオーバー時のカーソルはIビームに変更
            AssociatedObject.Cursor = Cursors.IBeam;      
        }

        private Window _window = null;

        private void AssociatedObject_Loaded(object sender, System.Windows.RoutedEventArgs e)
        {
            _window = Window.GetWindow(AssociatedObject);
            if (_window != null) {
                _window.MouseDown += _window_MouseDown;
                _window.PreviewKeyDown += _window_PreviewKeyDown;
            }
        }

        private void AssociatedObject_Unloaded(object sender, System.Windows.RoutedEventArgs e)
        {
            if (_window != null) {
                _window.MouseDown -= _window_MouseDown;
                _window.PreviewKeyDown -= _window_PreviewKeyDown;
            }
        }

        private void _window_MouseDown(object sender, MouseButtonEventArgs e)
        {
            if (e.ClickCount == 1 && StartSelectPosition != null && EndSelectPosition != null) {
                TextRange otr = new TextRange(AssociatedObject.ContentStart, AssociatedObject.ContentEnd);
                otr.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Colors.Transparent));
            }
        }

        private void _window_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            if ((e.Key == Key.C || e.ImeProcessedKey == Key.C) && Keyboard.Modifiers == ModifierKeys.Control) {
                // Ctrl+C が押された場合はクリップボードに選択内容をコピー
                if (!string.IsNullOrEmpty(SelectedText)) {
                    Clipboard.SetText(SelectedText);
                    e.Handled = true;
                    return;
                }
            }
        }

        private void AssociatedObject_MouseDown(object sender, MouseButtonEventArgs e)
        {
            if (e.ClickCount >= 2) {
                // ダブルクリック (全選択)
                SelectText(AssociatedObject.ContentStart, AssociatedObject.ContentEnd);
            } else {
                // シングルクリック (選択開始)
                if (AssociatedObject.CaptureMouse()) {
                    Point mouseDownPoint = e.GetPosition(AssociatedObject);
                    StartSelectPosition = AssociatedObject.GetPositionFromPoint(mouseDownPoint, true);
                    EndSelectPosition = StartSelectPosition;
                }
            }
        }

        private void AssociatedObject_MouseMove(object sender, MouseEventArgs e)
        {
            if (AssociatedObject.IsMouseCaptured) {
                if (StartSelectPosition == null)
                    return;
                // 選択中文字列の背景色変更
                Point mouseUpPoint = e.GetPosition(AssociatedObject);
                var newEndSelectPosition = AssociatedObject.GetPositionFromPoint(mouseUpPoint, true);
                TextRange ntr = new TextRange(StartSelectPosition, newEndSelectPosition);
                var text = ntr.Text;
                if (text.Length < SelectedText.Length && EndSelectPosition != null) {
                    TextRange etr = new TextRange(newEndSelectPosition, EndSelectPosition);
                    etr.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Colors.Transparent));
                }
                EndSelectPosition = newEndSelectPosition;
                ntr.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Colors.Violet));
                SelectedText = ntr.Text;
            }
        }

        private void AssociatedObject_MouseUp(object sender, MouseButtonEventArgs e)
        {
            if (AssociatedObject.IsMouseCaptured) {
                // 選択終了
                Point mouseUpPoint = e.GetPosition(AssociatedObject);
                EndSelectPosition = AssociatedObject.GetPositionFromPoint(mouseUpPoint, true);

                SelectText(StartSelectPosition, EndSelectPosition);

                AssociatedObject.ReleaseMouseCapture();
            }
        }

        private void SelectText(TextPointer start, TextPointer end)
        {
            // TextBlockから選択範囲文字列取得
            TextRange ntr = new TextRange(start, end);
            ntr.ApplyPropertyValue(TextElement.BackgroundProperty, new SolidColorBrush(Colors.Violet));
            SelectedText = ntr.Text;
        }
    }
}

これで以下のようにTextBlockで選択・コピーできるようになります。

XAMLで一々を指定したくない場合はスタイルでBehaviorを設定できる記事を誰かが公開してたと思いますので探してみてください。