Question

如果我能够通过Lambda表达式（下面）分配

，这将非常简单

//An expression tree cannot contain an assignment operator
Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[0].FirstName = "Tim";

由于赋值运算符，上面的代码无效。我需要传递一个lambda表达式，以便识别需要设置的复杂对象中的属性。在某些情况下，复杂对象具有List，因此有重复的对象类型和名称，这就是为什么我需要lambda显式引用要更新的对象中的字段。

我可以使用以下方法检索值，没问题。但我不知道如何使用相同的逻辑来设置值，我遇到了Expression.Assign，并相信这可能是解决方案。

Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[0].FirstName;
var result = FindByProperty(expression);

public static string FindByProperty(Expression<Func<Contract, object>> propertyRefExpr)
{
    ComplexObj obj = new ComplexObj();
    Contact myContact = new Contact();
    myContact.FirstName = "Allen";
    obj.Contacts = new List<Contact>{myContact};
    return propertyRefExpr.Compile().Invoke(obj);
}

更新

“将属性赋值作为表达式树传递给方法......”

对ParentTypeA使用SetValue方法，Value将不起作用。（下面的代码）

Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[0].FirstName;
obj.AssignNewValue(expression, firstName);

public static void AssignNewValue(this ComplexObj obj, Expression<Func<ComplexObj, object>> expression, object value)
{
    var propertyInfo = (PropertyInfo)((MemberExpression)expression.Body).Member;
    propertyInfo.SetValue(obj, value, null);
}

Answer 1

我最终使用了以下解决方案。干杯

ComplexObj obj = new ComplexObj();
Expression<Func<ComplexObj, object>> expression = obj => obj.Contacts[index].FirstName;
obj.AssignNewValue(expression, firstName);

public static void AssignNewValue(this ComplexObj obj, Expression<Func<ComplexObj, object>> expression, object value)
{
    ParameterExpression valueParameterExpression = Expression.Parameter(typeof(object));
    Expression targetExpression = expression.Body is UnaryExpression ? ((UnaryExpression)expression.Body).Operand : expression.Body;

    var newValue = Expression.Parameter(expression.Body.Type);
    var assign = Expression.Lambda<Action<ComplexObj, object>>
                (
                    Expression.Assign(targetExpression, Expression.Convert(valueParameterExpression, targetExpression.Type)),
                    expression.Parameters.Single(),
                    valueParameterExpression
                );

    assign.Compile().Invoke(obj, value);
}

Answer 2

链接问题可能是“正确”的答案，但只是为了完整起见，你可以做更多这样的事情......它确实遵循“告诉，不要”请问“方法有点好，虽然我不能说我喜欢实施......

void Main()
{
    Expression<Func<ComplexObj, object>> expression = 
      obj => obj.Contacts[0].SetFirstName("Tim");       
}

public class ComplexObj
{
    public ComplexObj() { Contacts = new List<SimpleObj>(); }
    public List<SimpleObj> Contacts {get; private set;}
}
public class SimpleObj
{
    public string FirstName {get; private set;}
    public SimpleObj SetFirstName(string name) { this.FirstName = name; return this; }
}

如何通过Expression分配值？

2 个答案: