Sharepoint Webpart

I am trying to access a Web service by using a Web Part. When I do so, I get a Security Exception as follows:

Request for the permission of type System.Net.WebPermission, System, 

  Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 

  failed.

By default, assemblies in the BIN directory do not have the required permission, System.Net.WebPermission, to access Web services. To grant this permission, add the following to the corresponding IPermission element in the appropriate policy file:

I want to access a Web service from my Web Part. When I do so, I get an InvalidOperationException as follows:

One or more assemblies referenced by the XmlSerializer cannot be called 

  from partially trusted code.

When you create a reference to a Web service, Microsoft Visual Studio®.NET creates and places one or more objects in your assembly to store the argument data passed to the method(s) in the Web service. These objects are serialized using the XmlSerializer class when you invoke one or more of the methods in the Web service. By default, if your assembly is strongly named and resides in the BIN directory, callers with partial trust cannot access objects within that assembly. When you make the call to the Web service method, the XmlSerializer detects that there are partially trusted callers on the callstack (i.e. your assembly) and prevents the serialization from taking place even though the object resides in the same assembly.

You have several possible solutions, both of which have security implications that you must consider:

1.     You can add the AllowPartiallyTrustedCallersAttribute attribute to the specified library assembly.

Important   You can only add this attribute to the source code. If you are using a third-party assembly and do not have access to the source, you cannot choose this option. If you choose this option, you are allowing partially trusted callers to execute code from within the library. This could represent a potential security risk as it opens the specified library assembly for use by other callers with partial trusts.

2.     You can give your assembly Full trust by installing it to the GAC.

Important   Assemblies installed in the GAC are available to all virtual servers and applications on the server running Windows SharePoint Services. This could represent a potential security risk as it potentially grants a higher level of permission to your assembly across a larger scope than necessary.

3.     You can give your assembly Full trust by creating a custom policy file as outlined in the previous section.

Important   It is recommended that you choose this option as it allows you to explicitly grant the required minimum level of permission to your assembly without increasing the scope of access to a larger number of callers.