返回语句后，XmlDocument变为null

Question

我正在制作一个HttpWebRequest，我将Xml文档发送到Web服务，并将请求的结果返回给单元测试。在return语句之后，Xml Document被取消了。为什么？

public class BaseTest
{
    public XmlDocument _xmlDoc; 

    public string CallWebService()
    {
    XmlDocument _xmlDoc = "./xmlfile.xml";

    /*Prepare HttpHeaders and make webservice call*/

    string result = reader.ReadToEnd().ToString();
    return result;//<-----_xmlDoc becomes null at this point. 

    }

}

[TestMethod]
public void TestWebService()
{
     string result = CallWebService();
     Assert.IsTrue(result.Contains("SUCCESS"));  
}

Answer 1

更改

XmlDocument _xmlDoc = "./xmlfile.xml";

到

_xmlDoc = "./xmlfile.xml";