Invalid WCF async service contract

InvalidAsyncServiceContract.cs

[ServiceContractAttribute(Namespace = "http://microsoft.samples")]
public interface ISampleService
{
    [OperationContractAttribute]
    string SampleMethod(string msg);

    [OperationContractAttribute(AsyncPattern = true)]
    IAsyncResult BeginSampleMethod(string msg, AsyncCallback callback, 
        object asyncState);

    string EndSampleMethod(IAsyncResult result);

    [OperationContractAttribute(Name = "SampleMethod")]
    Task<string> SampleMethodAsync(string msg);
}