如何从匿名Linq.Expressions.Expression返回单个值

Question

When using Entity Framework with LINQ I can code like:
var a = context.Employee.Where(x => x.Name.Equals("Michael")).FirstOrDefault();

I'm curious how to create custom method with anonymous linq expression. I have a model class and method as follow

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
}

static T Single<T>(Expression<Func<T, bool>> predicate) where T : class
{
    //This is from the database
    List<Employee> employees = new List<Employee>
    {
        new Employee { Id = 1, Name = "Michael" },
        new Employee { Id = 2, Name = "Derek" }
    };

    //I have no idea how to return single value of employees

    //var expression = (BinaryExpression)predicate.Body;
    //var left = expression.Left.Type.Name;
    //Func<T, bool> func = predicate.Compile();
    //T value = method(html.ViewData.Model);
    //T t = (T)Convert.ChangeType(a, typeof(T));
    //Employee emp = (Employee)Convert.ChangeType(t, typeof(Employee));

    //var body = predicate.Body;

    //var prop = (PropertyInfo)((MemberExpression)predicate.Body).Member;
    //var propValue = prop.GetValue(func, null);

    //var parameter = Expression.Parameter(typeof(T));
    //var property = Expression.Property(parameter, expression.Member.Name);
    //var equal = Expression.Equal(property, Expression.Constant(propValue));
    //var lambda = Expression.Lambda<Func<T, bool>>(equal, parameter);


    return null;
}

Calling the above method should be like

Employee emp = Single<Employee>(d => d.Name == "Michael");

Answer 1

You can call FirstOrDefault directly

var a = context.Employee.FirstOrDefault(x => x.Name.Equals("Michael")); 

Or inside your function you can use

return employees.FirstOrDefault(predicate);

Answer 2

Try to use IQuerable.Where instead of IEnumerable.Where First one uses Expression<Func<TSource, Boolean>> as parameter. Second uses Func<TSource, Boolean> .

So you need to use AsQuerable() and then Where() So in your case it should look like

return employees.AsQuerable().Where(predicate).FirstOrDefault()

If you want to pass delegate instead of Expression you need to change method to static T Single<T>(Func<T, bool> predicate) where T : class and then return employees.FirstOrDefault(predicate)

So you can use it this way

Employee emp = Single<Employee>(d => d.Name == "Michael");