Can I have an optional parameter for an ASP.NET SOAP web service

You can have a Overloaded Method in webservices with MessageName attribute. This is a workaround to achieve the overloading functionality.

Look at http://msdn.microsoft.com/en-us/library/byxd99hx%28VS.71%29.aspx

[WebMethod(MessageName="Add3")]
public double Add(double dValueOne, double dValueTwo, double dValueThree)
{
   return dValueOne + dValueTwo + dValueThree;
}

[WebMethod(MessageName="Add2")]
public int Add(double dValueOne, double dValueTwo)
{
   return dValueOne + dValueTwo;
}

The methods will be made visible as Add2 and Add3to the outside.

In accordance with MinOccurs Attribute Binding Support and Default Attribute Binding Support:

1. Value type accompanied by a public bool field that uses the Specified naming convention described previously under Translating XSD to source - minOccurs value of output <element> element 0.

   [WebMethod]
   public SomeResult SomeMethod(bool optionalParam, [XmlIgnore] bool optionalParamSpecified)

   result:

   <s:element minOccurs="0" maxOccurs="1" name="optionalParam" type="s:boolean" />

2. Value type with a default value specified via a System.Component.DefaultValueAttribute - minOccurs value of output <element> element 0. In the <element> element, the default value is also specified via the default XML attribute.

   [WebMethod]
   public SomeResult SomeMethod([DefaultValue(true)] bool optionalParam)

   result:

   <s:element minOccurs="0" maxOccurs="1" default="true" name="optionalParam" type="s:boolean" />

If you REALLY have to accomplish this, here's a sort of hack for the specific case of a web method that has only primitive types as parameters:

[WebMethod]
public void MyMethod(double requiredParam1, int requiredParam2)
{
    // Grab an optional param from the request.
    string optionalParam1 = this.Context.Request["optionalParam1"];

    // Grab another optional param from the request, this time a double.
    double optionalParam2;
    double.TryParse(this.Context.Request["optionalParam2"], out optionalParam2);
    ...
}

I know this post is little old. But I think the method names should be same for the example from Rasik. If both method names are same, then where overloading comes there. I think it should be like this:

[WebMethod(MessageName="Add3")]
public double Add(double dValueOne, double dValueTwo, double dValueThree)
{
   return dValueOne + dValueTwo + dValueThree;
}

[WebMethod(MessageName="Add2")]
public int Add(double dValueOne, double dValueTwo)
{
   return dValueOne + dValueTwo;
}