C# how to get my interface working?

Hi everyone, can someone tell me how I can properly get this to work? I'm having some trouble with WCF Web service using Interface.

Here's the solution structure:
- IPlugin.cs (contains the IService and Service1 classes) <--- service library
- OrderLibrary.cs (is a class library which will need to reference one of the methods (writefile) in the IPlugin.cs class)

Currently, the OrderLibrary is using a Reference (Microsoft...dll) so the public class header of the code looks like this:
public class Plugin : IPlugin
{
..............
}

I guess the part that's confusing is the function, I'm creating is a void.

This is the method I want to add to my code, but not sure how to add it. can someone help?

 void WriteFile(string filePath, string fileName)
        {
            if (File.Exists(fileName))
                File.Create(filePath + "\\" + fileName + DateTime.Now.ToShortDateString());
            else
                File.Create(filePath + "\\" + fileName);
        }

Select allOpen in new window

   

IService1.cs

  public interface IService1
    {
        [OperationContract]
        string GetData(int value);

        [OperationContract]
        CompositeType GetDataUsingDataContract(CompositeType composite);

        // TODO: Add your service operations here
         
   }
 public class CompositeType
    {
        bool boolValue = true;
        string stringValue = "Hello ";

        [DataMember]
        public bool BoolValue
        {
            get { return boolValue; }
            set { boolValue = value; }
        }

        [DataMember]
        public string StringValue
        {
            get { return stringValue; }
            set { stringValue = value; }
        }
       
   }
    
   

Select allOpen in new window

 public class Service1 : IService1
    {
        public string GetData(int value)
        {
            return string.Format("You entered: {0}", value);
        }

        public CompositeType GetDataUsingDataContract(CompositeType composite)
        {
            if (composite == null)
            {
                throw new ArgumentNullException("composite");
            }
            if (composite.BoolValue)
            {
                composite.StringValue += "Suffix";
            }
            return composite;
        }

        public writeFileClass wf(writeFileClass wf)
        {

            return wf;
        }

      
         
    
    } 
    



    }
 

Select allOpen in new window