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In a Visual Studio .NET project, right click the project references and choose 'Add Service Reference', enter the URL of JIRA's WSDL descriptor (http://your_installation/rpc/soap/jiraservice-v1.wsdl), and Visual Studio will auto-generate a .NET class for accessing the JIRA SOAP API.
The parameter names aren't particularly meaningful so you'll need to refer back to the documentation quite a bit at first.