Windows表单中的TextBoxAutoComplete如果在数据库中没有匹配的值，则显示默认值。

可以通过在文本框的LostFocus事件中执行以下代码来实现此功能：

1. 定义一个默认值变量

string defaultValue = "未找到匹配项";

2. 查询数据库并查找匹配项

string searchTerm = textBox1.Text;
string matchedValue = null;
// 连接数据库并执行查询
using (SqlConnection connection = new SqlConnection("connectionString"))
{
    string query = "SELECT * FROM table WHERE Column LIKE @SearchTerm";
    SqlCommand command = new SqlCommand(query, connection);
    command.Parameters.AddWithValue("@SearchTerm", searchTerm + "%");
    connection.Open();
    SqlDataReader reader = command.ExecuteReader();
    if (reader.Read())
    {
        matchedValue = reader["Column"].ToString();
    }
    reader.Close();
}

3. 根据匹配项更新文本框

if (matchedValue != null)
{
    textBox1.Text = matchedValue;
}
else
{
    textBox1.Text = defaultValue;
}

完整的代码示例：

string defaultValue = "未找到匹配项";

private void textBox1_LostFocus(object sender, EventArgs e)
{
    string searchTerm = textBox1.Text;
    string matchedValue = null;
    // 连接数据库并执行查询
    using (SqlConnection connection = new SqlConnection("connectionString"))
    {
        string query = "SELECT * FROM table WHERE Column LIKE @SearchTerm";
        SqlCommand command = new SqlCommand(query, connection);
        command.Parameters.AddWithValue("@SearchTerm", searchTerm + "%");
        connection.Open();
        SqlDataReader reader = command.ExecuteReader();
        if (reader.Read())
        {
            matchedValue = reader["Column"].ToString();
        }
        reader.Close();
    }
    
    if (matchedValue != null)
    {
        textBox1.Text = matchedValue;
    }
    else
    {
        textBox1.Text = defaultValue;
    }
}