BUG: "Cannot implicitly convert type 'System.Data.DataSet'" error when application is built

1. In Solution Explorer, click the Web Application project.
2. On the Project menu, click Show All Files.
3. In Solution Explorer, expand Web References, and then expand localhost.
4. Expand the Reference.map node, and then double-click the proxy file Reference.cs or Reference.vb.
5. Locate the method that incorrectly returns System.Data.DataSet. For example, locate the following code for the proxy file of the Web Service that is used in the "More Information" section of this article:

   Visual C# .NET Code

           [System.Web.Services.Protocols.SoapDocumentMethodAttribute("http://tempuri.org/GetMyObject", RequestNamespace="http://tempuri.org/", ResponseNamespace="http://tempuri.org/", Use=System.Web.Services.Description.SoapBindingUse.Literal, ParameterStyle=System.Web.Services.Protocols.SoapParameterStyle.Wrapped)]
           public System.Data.DataSet GetMyObject() {
               object[] results = this.Invoke("GetMyObject", new object[0]);
               return ((System.Data.DataSet)(results[0]));
           }

   Visual Basic .NET Code

         <System.Web.Services.Protocols.SoapDocumentMethodAttribute("http://tempuri.org/GetMyObject", RequestNamespace:="http://tempuri.org/", ResponseNamespace:="http://tempuri.org/", Use:=System.Web.Services.Description.SoapBindingUse.Literal, ParameterStyle:=System.Web.Services.Protocols.SoapParameterStyle.Wrapped)> _
         Public Function GetMyObject() As System.Data.DataSet
            Dim results() As Object = Me.Invoke("GetMyObject", New Object(-1) {})
            Return CType(results(0), System.Data.DataSet)
         End Function

6. Manually replace System.Data.DataSet with the name of your custom class that implements the IXmlSerializable interface. For example, modify the method that you located in step 5 as follows:

   Visual C# .NET Code

           [System.Web.Services.Protocols.SoapDocumentMethodAttribute("http://tempuri.org/GetMyObject", RequestNamespace="http://tempuri.org/", ResponseNamespace="http://tempuri.org/", Use=System.Web.Services.Description.SoapBindingUse.Literal, ParameterStyle=System.Web.Services.Protocols.SoapParameterStyle.Wrapped)]
           public MyTestClass GetMyObject() {
               object[] results = this.Invoke("GetMyObject", new object[0]);
               return ((MyTestClass)(results[0]));
           }

   Visual Basic .NET Code

         <System.Web.Services.Protocols.SoapDocumentMethodAttribute("http://tempuri.org/GetMyObject", RequestNamespace:="http://tempuri.org/", ResponseNamespace:="http://tempuri.org/", Use:=System.Web.Services.Description.SoapBindingUse.Literal, ParameterStyle:=System.Web.Services.Protocols.SoapParameterStyle.Wrapped)> _
         Public Function GetMyObject() As MyTestClass
            Dim results() As Object = Me.Invoke("GetMyObject", New Object(-1) {})
            Return CType(results(0), MyTestClass)
         End Function

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Steps to Reproduce the Behavior

1. Create a new Visual Basic or Visual C# ASP.NET Web Service project, and name it MyTestWebService.
2. In Solution Explorer, right-click Service1.asmx, and then click View Code.
3. Replace the existing code with the following code:
   

   Visual C# .NET Code

   using System;
   using System.Collections;
   using System.ComponentModel;
   using System.Data;
   using System.Diagnostics;
   using System.Web;
   using System.Web.Services;
   using System.Xml;
   using System.Xml.Schema;
   using System.Xml.Serialization;
   
   namespace MyTestWebService
   {
      public class MyTestWebService : WebService
      {
         public MyTestWebService()
         {
         }
         
         // Web Service method that Returns Custom Class
         [WebMethod]
         public MyTestClass GetMyObject()
         {
            MyTestClass myObj = new MyTestClass();
            return myObj;
         }
      }
   
      // Class that implements IXmlSerializable
      public class MyTestClass : IXmlSerializable
      {
         // Constructor
         public MyTestClass()
         {
         }
         
         // Implementing IXmlSerializable method GetSchema 
         XmlSchema IXmlSerializable.GetSchema()
         {
            return null;
         }
         
         // Implementing IXmlSerializable method ReadXml
         void IXmlSerializable.ReadXml(XmlReader reader)
         {
         }
   
         // Implementing IXmlSerializable method WriteXml
         void IXmlSerializable.WriteXml(XmlWriter writer)
         {
         }
      }
   }

   Visual Basic .NET Code

   Imports System
   Imports System.Collections
   Imports System.ComponentModel
   Imports System.Data
   Imports System.Diagnostics
   Imports System.Web
   Imports System.Web.Services
   Imports System.Xml
   Imports System.Xml.Schema
   Imports System.Xml.Serialization
   
   <WebService(Namespace:="http://tempuri.org/")> _
   Public Class MyWebServiceClass
      Inherits System.Web.Services.WebService
   
      Public Sub New()
   
      End Sub
   
      <WebMethod()> _
      Public Function GetMyObject() As MyTestClass
         Dim myObj As New MyTestClass()
         Return myObj
      End Function
   
   End Class
   
   ' Class that implements IXmlSerializable
   Public Class MyTestClass
      Implements IXmlSerializable
   
      ' Default Constructor
      Public Sub New()
   
      End Sub
   
      ' Implementing IXmlSerializable method GetSchema 
      Public Function GetSchema() As XmlSchema Implements IXmlSerializable.GetSchema
   
      End Function
   
      ' Implementing IXmlSerializable method ReadXml
      Public Sub ReadXml(ByVal reader As XmlReader) Implements IXmlSerializable.ReadXml
   
      End Sub
   
      ' Implementing IXmlSerializable method WriteXml
      Public Sub WriteXml(ByVal writer As XmlWriter) Implements IXmlSerializable.WriteXml
   
      End Sub
   End Class

4. On the Build menu, click Build Solution.
5. On the File menu, point to Add Project, and then click New Project.
6. Under Project Types, click Visual C# Projects or Visual Basic Projects, and under Templates, click ASP.NET Web Application.
7. Name the project MyTestApplication, and then click OK.
8. In Solution Explorer, right-click WebForm1.aspx, and then click View Code.
9. Replace the existing code with the following code:

   Visual C# .NET Code

   using System;
   using System.Collections;
   using System.ComponentModel;
   using System.Drawing;
   using System.Web;
   using System.Web.SessionState;
   using System.Web.UI;
   using System.Web.UI.WebControls;
   using System.Web.UI.HtmlControls;
   using System.Xml;
   using System.Xml.Schema;
   using System.Xml.Serialization;
   
   namespace WebApplication47
   {
   	public class WebForm1 : System.Web.UI.Page
   	{
   		private void Page_Load(object sender, System.EventArgs e)
   		{
   			localhost.MyObjectService obj = new localhost.MyObjectService();
            MyTestClass objTest;
            objTest = obj.GetMyObject();
   		}
   
   		#region Web Form Designer generated code
   		override protected void OnInit(EventArgs e)
   		{
   			InitializeComponent();
   			base.OnInit(e);
   		}
   		
   		private void InitializeComponent()
   		{    
   			this.Load += new System.EventHandler(this.Page_Load);
   		}
   		#endregion
   	}
   
      // Class that implements IXmlSerializable
      public class MyTestClass : IXmlSerializable
      {
         // Constructor
         public MyTestClass()
         {
         }
         
         // Implementing IXmlSerializable method GetSchema 
         XmlSchema IXmlSerializable.GetSchema()
         {
            return null;
         }
         
         // Implementing IXmlSerializable method ReadXml
         void IXmlSerializable.ReadXml(XmlReader reader)
         {
         }
   
         // Implementing IXmlSerializable method WriteXml
         void IXmlSerializable.WriteXml(XmlWriter writer)
         {
         }
      }
   }

   Visual Basic .NET Code

   Imports System
   Imports System.Xml
   Imports System.Xml.Schema
   Imports System.Xml.Serialization
   
   Public Class WebForm1
      Inherits System.Web.UI.Page
   
   #Region " Web Form Designer Generated Code "
   
      'This call is required by the Web Form Designer.
      <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
   
      End Sub
   
      Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
          InitializeComponent()
      End Sub
   
   #End Region
   
      Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
         Dim obj As New localhost.MyWebServiceClass()
         Dim objTest As MyTestClass
         objTest = obj.GetMyObject()
      End Sub
   
   End Class
   
   ' Class that implements IXmlSerializable
   Public Class MyTestClass
      Implements IXmlSerializable
   
      ' Default Constructor
      Public Sub New()
   
      End Sub
   
      ' Implementing IXmlSerializable method GetSchema 
      Public Function GetSchema() As System.Xml.Schema.XmlSchema Implements IXmlSerializable.GetSchema
   
      End Function
   
      ' Implementing IXmlSerializable method ReadXml
      Public Sub ReadXml(ByVal reader As System.Xml.XmlReader) Implements IXmlSerializable.ReadXml
   
      End Sub
   
      ' Implementing IXmlSerializable method WriteXml
      Public Sub WriteXml(ByVal writer As System.Xml.XmlWriter) Implements IXmlSerializable.WriteXml
   
      End Sub
   End Class

10. In Solution Explorer, right-click MyTestApplication, and then click Add Web Reference.
11. In the Address text box, type http://localhost/MyTestWebService/Service1.asmx, and then press ENTER.
12. Click Add Reference.
13. On the File menu, click Save All.
14. On the Build menu, click Build Solution. During compilation of MyTestApplication, you receive the error that is described in the "Symptoms" section.

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