<UserControl x:Class="WpfApplication18.SearchControl"
       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"
       mc:Ignorable="d" MinHeight="30" MinWidth="150" Background="Transparent"
       d:DesignHeight="30" d:DesignWidth="150">
  <Grid>
    <Grid.ColumnDefinitions>
      <ColumnDefinition Width="15"></ColumnDefinition>
      <ColumnDefinition Width="*"></ColumnDefinition>
      <ColumnDefinition Width="36"></ColumnDefinition>
    </Grid.ColumnDefinitions>
    <Rectangle Grid.Column="0" Grid.ColumnSpan="3" Fill="LightGray" RadiusX="15" RadiusY="15" Opacity="0.8"></Rectangle>
     
    <TextBox x:Name="TbxInput" Grid.Column="1" KeyDown="TbxInput_OnKeyDown">
      <TextBox.Template>
        <ControlTemplate TargetType="TextBox">
          <Grid>
            <TextBlock x:Name="Uc_TblShow" Text="请输入..." Foreground="Gray" Opacity="0.5" VerticalAlignment="Center" Visibility="Collapsed"></TextBlock>
            <TextBox x:Name="Uc_TbxContent" Foreground="Gray" Background="Transparent" VerticalAlignment="Center" VerticalContentAlignment="Center" BorderThickness="0"
                 Text="{Binding ElementName=TbxInput,Path=Text,Mode=TwoWay}" FontSize="18"></TextBox>
          </Grid>
          <ControlTemplate.Triggers>
            <Trigger SourceName="Uc_TbxContent" Property="Text" Value="">
              <Setter TargetName="Uc_TblShow" Property="Visibility" Value="Visible"></Setter>
            </Trigger>
            <Trigger SourceName="Uc_TbxContent" Property="IsFocused" Value="True">
              <Setter TargetName="Uc_TblShow" Property="Visibility" Value="Collapsed"></Setter>
            </Trigger>
          </ControlTemplate.Triggers>
        </ControlTemplate>
      </TextBox.Template>
    </TextBox>
     
    <Button x:Name="BtnSearch" Grid.Column="2" Click="BtnSearch_OnClick" Cursor="Hand">
      <Button.Template>
        <ControlTemplate TargetType="Button">
          <Grid>
            <Image x:Name="Uc_Image" Source="1181298.png" Height="20" Width="20"></Image>
            <ContentPresenter></ContentPresenter>
          </Grid>
          <ControlTemplate.Triggers>
            <Trigger Property="IsMouseOver" Value="true">
              <Setter TargetName="Uc_Image" Property="Height" Value="25"></Setter>
              <Setter TargetName="Uc_Image" Property="Width" Value="25"></Setter>
            </Trigger>
          </ControlTemplate.Triggers>
        </ControlTemplate>
      </Button.Template>
    </Button>
  </Grid>
</UserControl>

public partial class SearchControl : UserControl
{
  public SearchControl()
  {
    InitializeComponent();
  }
 
  public event EventHandler<SearchEventArgs> OnSearch; 
  private void BtnSearch_OnClick(object sender, RoutedEventArgs e)
  {
    ExeccuteSearch();
  }
 
  private void TbxInput_OnKeyDown(object sender, KeyEventArgs e)
  {
    ExeccuteSearch();
  }
 
  private void ExeccuteSearch()
  {
    if (OnSearch!=null)
    {
      var args=new SearchEventArgs();
      args.SearchText = TbxInput.Text;
      OnSearch(this, args);
    }
  }
}
public class SearchEventArgs : EventArgs
{
  public string SearchText { get; set; }
}