Linq to Entites Issue

Question

I am trying to use dynamic Linq in my project but have following Issue.

Invoice Class

public string ClientName { get; set; }
public TransactionType TransactionType { get; set; }

TransactionType Enum

public enum TransactionType
    {
        ServiceInvoice = 0,
        CreditInvoice = 1,
        Payment=2
    }

Simple Linq is working like excepted.

var incoicestest2 = _unitOfWork.GetRepository<Invoice>().Select().Where(x => x.TransactionType.ToString().Contains("ServiceIn"));//Working

but Dynamic Linq is not working

var SearchString="TransactionType.ToString().Contains (\"ServiceIn\")";

var incoicestest2 = _unitOfWork.GetRepository<Invoice>().Select()
            .Where(SearchString)
            .ToList(); //Not Working

And Error "LINQ to Entities does not recognize the method 'System.String ToString()' method,..."

I need to use Dynamic Linq because the searchString is created at run time.

Someone Help ,Please