Set up a WPF component for a text box with a placeholder.

FinModelUtility/UniversalAssetTool/UniversalAssetTool.Ui/src/ui/wpf/common/TextBoxWithPlaceholder.xaml

@@ -0,0 +1,23 @@
+<UserControl x:Class="uni.src.ui.wpf.common.TextBoxWithPlaceholder"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:local="clr-namespace:uni.src.ui.wpf.common"
+             mc:Ignorable="d"
+             d:DesignHeight="24"
+             d:DesignWidth="200">
+  <UserControl.DataContext>
+    <local:TextBoxWithPlaceholderViewModel />
+  </UserControl.DataContext>
+
+  <Grid>
+    <TextBox x:Name="impl_"
+             TextChanged="TextChanged_" />
+    <Label x:Name="placeholderLabel_"
+           Content="{Binding Placeholder }"
+           Background="Transparent"
+           Opacity="0.5"
+           IsHitTestVisible="False" />
+  </Grid>
+</UserControl>

FinModelUtility/UniversalAssetTool/UniversalAssetTool.Ui/src/ui/wpf/common/TextBoxWithPlaceholder.xaml.cs

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+using System.ComponentModel;
+using System.Windows.Controls;
+using System.Windows;
+
+namespace uni.src.ui.wpf.common {
+  /// <summary>
+  /// Interaction logic for WatermarkTextBox.xaml
+  /// </summary>
+  public partial class TextBoxWithPlaceholder : UserControl {
+    private readonly TextBoxWithPlaceholderViewModel viewModel_;
+
+    public TextBoxWithPlaceholder(TextBoxWithPlaceholderViewModel viewModel) {
+      InitializeComponent();
+      this.viewModel_ = viewModel;
+    }
+
+    public string Placeholder {
+      get => this.viewModel_.Placeholder;
+      set => this.viewModel_.Placeholder = value;
+    }
+
+    private void TextChanged_(object sender, TextChangedEventArgs args)
+      => this.placeholderLabel_.Visibility
+          = this.impl_.Text != ""
+              ? Visibility.Hidden
+              : Visibility.Visible;
+  }
+
+  public class TextBoxWithPlaceholderViewModel : INotifyPropertyChanged {
+    private string placeholder_ = "Search...";
+    public event PropertyChangedEventHandler? PropertyChanged;
+
+    public string Placeholder {
+      get => this.placeholder_;
+      set {
+        this.placeholder_ = value;
+        this.PropertyChanged?.Invoke(
+            this,
+            new PropertyChangedEventArgs(nameof(Placeholder)));
+      }
+    }
+  }
+}