如何使用 Expression<Func> 设置嵌套属性？

Question

所以我有一些代码可以设置对象的属性。此代码来自我们在单元测试中使用的内部验证类。所以代码可能会提供类似

private static void SetDeepValue(object targetObject, Expression<Func<string>> propertyToSet, object valueToSet)
        {
            var underlyingProperty = ((PropertyInfo)((MemberExpression)propertyToSet.Body).Member);
            underlyingProperty.SetValue(targetObject, valueToSet);
        }

此代码用于单元测试类型的环境，然后我们可以在其中进行类似的调用

foreach (string currentTestCaseValue in TestCaseSets)
{
     BusinessObject myCustomer = new BusinessObject();
     SetDeepValue(myCustomer, ()=>myCustomer.FirstName,currentTestCaseValue);
     ValidateBusinessRules(myCustomer);
}

（为了简洁/复杂而简化的代码）

然而，现在，由于一些重构，我们得到了类似的东西：

foreach (string currentTestCaseValue in TestCaseSets)
    {
         BusinessObject myCustomer = new BusinessObject();
         SetDeepValue(myCustomer, ()=>myCustomer.NameInfo.First,currentTestCaseValue);
         ValidateBusinessRules(myCustomer);
    }

当这段代码运行时，我们得到错误：

对象与目标类型不匹配。

我怀疑它试图调用BusinessObject 上的First 属性，而不是NameInfo。如何修改我的代码来处理这种“嵌套”情况？

Answer 1

以下是您通常如何将字符串 "ColumnName1.ColumnName2" 转换为 lambda 表达式 x => x.ColumnName1.ColumnName2：

Expression<Func<T, object>> ForNestedProperty(string columnName)
{
    // x
    ParameterExpression param = Expression.Parameter(typeof(T), "x");

    // x.ColumnName1.ColumnName2
    Expression property = columnName.Split('.')
                                    .Aggregate<string, Expression>
                                    (param, (c, m) => Expression.Property(c, m));

    // x => x.ColumnName1.ColumnName2
    Expression<Func<T, object>> lambda = Expression.Lambda<Func<T, object>>(
        Expression.Convert(property, typeof(object)), param);
    return lambda;
}

（复制自here）

Answer 2

现在您已经给我们举了一个例子，这很容易做到。以任何方式编译表达式都是没有用的，因为我们不能重用它，所以它只会减慢方法。更容易走 getter 的“链”并使用反射来访问它们的值。我写的方法同时支持字段（通常用作readonly字段）和属性。

public static void SetDeepValue<T>(object notUsed, Expression<Func<T>> propertyToSet, T valueToSet)
{
    List<MemberInfo> members = new List<MemberInfo>();

    Expression exp = propertyToSet.Body;
    ConstantExpression ce = null;

    // There is a chain of getters in propertyToSet, with at the 
    // beginning a ConstantExpression. We put the MemberInfo of
    // these getters in members and the ConstantExpression in ce

    while (exp != null)
    {
        MemberExpression mi = exp as MemberExpression;

        if (mi != null)
        {
            members.Add(mi.Member);
            exp = mi.Expression;
        }
        else
        {
            ce = exp as ConstantExpression;

            if (ce == null)
            {
                // We support only a ConstantExpression at the base
                // no function call like
                // () => myfunc().A.B.C
                throw new NotSupportedException();
            }

            break;
        }
    }

    if (members.Count == 0)
    {
        // We need at least a getter
        throw new NotSupportedException();
    }

    // Now we must walk the getters (excluding the last).
    // From the ConstantValue ce we take the base object
    object targetObject = ce.Value;

    // We have to walk the getters from last (most inner) to second
    // (the first one is the one we have to use as a setter)
    for (int i = members.Count - 1; i >= 1; i--)
    {
        PropertyInfo pi = members[i] as PropertyInfo;

        if (pi != null)
        {
            targetObject = pi.GetValue(targetObject);
        }
        else
        {
            FieldInfo fi = (FieldInfo)members[i];
            targetObject = fi.GetValue(targetObject);
        }
    }

    // The first one is the getter we treat as a setter
    {
        PropertyInfo pi = members[0] as PropertyInfo;

        if (pi != null)
        {
            pi.SetValue(targetObject, valueToSet);
        }
        else
        {
            FieldInfo fi = (FieldInfo)members[0];
            fi.SetValue(targetObject, valueToSet);
        }
    }
}

你可以这样使用它：

A a = new A();

SetDeepValue(a, () => a.B.C.Value, "Foo");

请注意，SetDeepValue 不需要也不会使用 targetObject，因为它可以在 getter 链中发现它：

SetDeepValue(myCustomer, ()=>myCustomer.FirstName, currentTestCaseValue);

这里有()=>myCustomer。

如果你调用的格式是这样的，那将是必要的

SetDeepValue(myCustomer, x=>x.FirstName, currentTestCaseValue);

我什至会给你一个使用Expression的第二种格式的方法：

public static void SetDeepValue<TObject, T>(TObject target, Expression<Func<TObject, T>> propertyToSet, T valueToSet)
{
    List<MemberInfo> members = new List<MemberInfo>();

    Expression exp = propertyToSet.Body;

    // There is a chain of getters in propertyToSet, with at the 
    // beginning a ParameterExpression. We put the MemberInfo of
    // these getters in members. We don't really need the 
    // ParameterExpression

    while (exp != null)
    {
        MemberExpression mi = exp as MemberExpression;

        if (mi != null)
        {
            members.Add(mi.Member);
            exp = mi.Expression;
        }
        else
        {
            ParameterExpression pe = exp as ParameterExpression;

            if (pe == null)
            {
                // We support only a ParameterExpression at the base
                throw new NotSupportedException();
            }

            break;
        }
    }

    if (members.Count == 0)
    {
        // We need at least a getter
        throw new NotSupportedException();
    }

    // Now we must walk the getters (excluding the last).
    object targetObject = target;

    // We have to walk the getters from last (most inner) to second
    // (the first one is the one we have to use as a setter)
    for (int i = members.Count - 1; i >= 1; i--)
    {
        PropertyInfo pi = members[i] as PropertyInfo;

        if (pi != null)
        {
            targetObject = pi.GetValue(targetObject);
        }
        else
        {
            FieldInfo fi = (FieldInfo)members[i];
            targetObject = fi.GetValue(targetObject);
        }
    }

    // The first one is the getter we treat as a setter
    {
        PropertyInfo pi = members[0] as PropertyInfo;

        if (pi != null)
        {
            pi.SetValue(targetObject, valueToSet);
        }
        else
        {
            FieldInfo fi = (FieldInfo)members[0];
            fi.SetValue(targetObject, valueToSet);
        }
    }
}

您可以比较两者以查看差异。