我在使用此 linq 时遇到问题

Question

I need to get some data out and use it in other level. I can't return (c), and I'm looking for a pattern. How can I return that correctly?

public string AutoUpdate(string _search)
{
    using (var context = new Phone_BookEntities1())
    {
        var c = from d in context.Cantacts
                where d.Cantact1 == _search
                select d.Cantact1.ToString();
        return c;
    }
}

Answer 1

The Problem is your query is returning IEnumerable<T> where T is the type of Cantact1 but here since you are converting it to string, it will return IEnumerable<string> . But since the return type is string you need to return single item. You can use FirstOrDefault :-

 var c = (from d in context.Cantacts
          where d.Cantact1 == _search
          select d.Cantact1.ToString()).FirstOrDefault();

Or with Lambda syntax:-

var c = context.Cantacts.FirstOrDefault(d => d.Cantact1 == _search);
if(c != null)
   return c.Cantact1.ToString();
else
   return String.Empty;  //any default value

Answer 2

I wanted to reiterate Rahul's solution, and expand a bit on the Lambda solution.

The following demonstrates the declaration and usage of Lambda Expressions.

First, ensure you have these namespaces:

using System;
using System.Linq;
using System.Linq.Expressions;

then

public string AutoUpdate(string _search)
{

    Expression<Func<Cantact, bool>> WhereClause = (d) => d.Cantact1 == _search;
    // thats: System.Linq.Expressions.Expression<System.Func<Cantact, bool>>

    return FirstContact(WhereClause);
    // or
    //return FirstContact((d) => d.Cantact1 == _search));
}

public string FirstContact(Expression<Func<Cantact, bool>> WhereClause) 
{
    using (var context = new Phone_BookEntities1()) 
    {
        //var query = context.Cantacts.FirstOrDefault(WhereClause);
        var query = context.Cantacts.Where(WhereClause).FirstOrDefault();            
        return (query == null) ? String.Empty : query.Cantact1;
    }
}

If _search where a static or constant we wouldn't have to declare WhereClause in the function scope.

As you can see in the code snippet, we moved the functionality of getting the First Contact into it's own function, and AutoUpdate just builds the expression and passes it..

---

Note:

You're returning query.Cantact1 , which is going to be _search .. You should probably consider returning the Model.Cantact entity, or if AutoUpdate is supposed to update entities at some point, return bool or just make the method void , following a Query Command Separation (QCS) Pattern/Responsibility.