为什么程序运行过程中提示运行报错 <b> ?报错

在VS2008调试环境下string提示是一个类类型，不能用做表达式，提示报错。

Partial Class _Default
Inherits System.Web.UI.Page

Protected Sub TextBox1_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles txtName.TextChanged

End Sub

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
    String  strHtml = "";
    strHtml += txtName.value + "<br/>";
    strHtml += txtStreet.value +"<br/>";
    strHtml += txtCity.value +"<br/>";
    tdInnerHtml.innerHtml = strHtml;
End Sub

End Class