如何从匿名Linq.Expressions.Expression返回单个值

本文关键字：Expressions Expression 返回 单个值 Linq | 更新日期: 2023-09-27 18:09:11

当使用LINQ实体框架时，我可以这样写:
var a = context.Employee.Where(x => x.Name.Equals("Michael")).FirstOrDefault();

我很好奇如何使用匿名linq表达式创建自定义方法。我有一个模型类和方法如下

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;
}

调用上面的方法应该像

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

如何从匿名Linq.Expressions.Expression返回单个值

可以直接调用FirstOrDefault 

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

或者在函数内部使用

return employees.FirstOrDefault(predicate);

尝试使用IQuerable.Where代替IEnumerable.Where第一个使用Expression<Func<TSource, Boolean>>作为参数。第二个使用Func<TSource, Boolean>。

所以你需要使用AsQuerable()，然后Where()在你的例子中应该是

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

如果您想传递delegate而不是Expression，则需要将method更改为static T Single<T>(Func<T, bool> predicate) where T : class，然后return employees.FirstOrDefault(predicate)

所以你可以这样使用

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

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