访问表达式主体成员以构建表达式树 [英] Accessing expression bodied members to build expression trees

问题描述

尝试使用表达式树通过表达式构建订单.但是我无法访问查询结果的类的表达式主体属性. 这是类结构:

Trying to build an order by expression using expression trees. But I am unable to access an expression bodied property of the query result's class. This is the class structure:

public class AssetFileRecord : IAuditable, IEntity, INavigateToCustomValues
{
    public AssetFileRecord()
    {
        this.UpdatedTimeStamp = DateTime.UtcNow;
    }

    public AssetFileRecord GetRecord()
    {
        return this;
    }

    public Guid Id { get; set; }
    public int DisplayId { get; set; }
    public string AssetTagNumber { get; set; }
    [JObjectIgnore]
    public virtual Account Account { get; set; }
    public string AccountNumber => Account?.AccountNumber;
    public string AuditTrail { get; set; }
    public string OldTagNumber { get; set; }
    public ActivityCode ActivityCode { get; set; }

    [JObjectIgnore]
    public virtual ICollection<AssetFileRecordDepreciation> AssetFileRecordDepreciations { get; set; }
    // Depreciation Records
    public double? AccumulatedDepreciation => Depreciation()?.AccumulatedDepreciation;
    public DateTime? DepreciationAsOfDate => Depreciation()?.DepreciationAsOfDate;
    public double? LifeMonths => Depreciation()?.LifeMonths;
    public double? DepreciationBasis => Depreciation()?.DepreciationBasis;
    public double? PeriodDepreciation => Depreciation()?.PeriodDepreciation;

    private AssetFileRecordDepreciation Depreciation()
    {
        return AssetFileRecordDepreciations?.AsQueryable()?.OrderBy(d => d.AssetFileDepreciationBook.BookNo)?.FirstOrDefault();
    }
}

我无法访问属性AccountNumber，它是AssetFileRecord的虚拟属性的属性.

I am unable to get to the property AccountNumber which is a property of a virtual property of AssetFileRecord.

以下是当前代码，该代码对于任何其他非表达主体属性都适用.

Below is the current code that works fine for any other non-expression bodied properties.

var type = typeof(T);
var property = type.GetProperty(sortProperty, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);
var parameter = Expression.Parameter(type, "p");
var propertyAccess = Expression.MakeMemberAccess(parameter, property);
var orderByExp = Expression.Lambda(propertyAccess, parameter);
var typeArguments = new[] { type, property.PropertyType };
var methodBase = isFirstOrderTerm ? "OrderBy" : "ThenBy";
var methodName = sortOrder == ListSortDirection.Ascending ? methodBase : $"{methodBase}Descending";
var resultExp = Expression.Call(typeof(Queryable), methodName, typeArguments, source.Expression, Expression.Quote(orderByExp));

return source.Provider.CreateQuery<T>(resultExp);

Expression.Call不会评估为有效的SQL查询，而是会引发异常.

Expression.Call does not evaluate to a valid SQL query and rather throws an exception.

((System.Data.Entity.Infrastructure.DbQuery<AssetFileRecord>)records).Sql = '((System.Data.Entity.Infrastructure.DbQuery<AssetFileRecord>)records).Sql' threw an exception of type 'System.NotSupportedException'

预期结果:应将按表达式的顺序附加到最后生成的表达式树中；尝试通过表达式属性属性成员进行排序时，这样做会失败.

Intended result: It should append an order by expression to the expression tree generated in the end; while it's failing to do so, when tried to order by an expression bodied property member.

有人可以帮我解决这个问题.

Can someone please help me get this working.

推荐答案

您的方法有两个问题.首先是您不能在Linq Expressions中使用null传播运算符.测试此代码:

You have two problems with your approach. First is that you can't use a null propagating operator in Linq Expressions. Test this code:

var account = new Account();
// will cause "error CS8072: An expression tree lambda may not contain a null propagating operator"    
Expression<Func<string>> accountNumber = () => account?.AccountNumber;

第二个主要问题是您的AccountNumber将被编译为get_AccountNumber方法，并且您无法使用Linq to SQL调用任意方法.您可以测试以下代码:

Second and main problem is that your AccountNumber will be compiled into get_AccountNumber method and you can't invoke arbitrary methods with Linq to SQL. You may test this code:

public class AssetFileRecord
{
  //...
  public string AccountNumber => Account != null ? Account.AccountNumber : null;
}

虽然可以编译，但会产生相同的运行时异常.

while this can be compiled, it produce same runtime exception.

解决此问题的一种可能方法是使用复杂属性的表达式创建映射:

One possible way to workaround this problem is to create a map with expressions for complex properties:

var map = new Dictionary<string, Expression>
{
    {
        "AssetFileRecord.AccountNumber", // type and property
        (Expression<Func<AssetFileRecord, string>>) (
            afr => afr.Account != null ? afr.Account.AccountNumber : null
        )
    }
};

现在，您可以重写针对此地图构建动态OrderBy的方法:

Now you can rewrite your method that builds dynamic OrderBy with respect to this map:

private static IQueryable<T> DynamicOrderBy<T>(
    IQueryable<T> source,
    string sortProperty,
    Dictionary<string, Expression> map)
{
    var type = typeof(T);
    var parameter = Expression.Parameter(type, "p");
    var property = type.GetProperty(sortProperty, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);

    Expression whereLambda;
    if (!map.TryGetValue($"{type.Name}.{sortProperty}", out whereLambda))
    {
        var propertyAccess = Expression.MakeMemberAccess(parameter, property);
        whereLambda = Expression.Lambda(propertyAccess, parameter);
    }
    // else we just using a lambda from map

    // call OrderBy
    var query = Expression.Call(
        typeof(Queryable),
        "OrderBy",
        new[] {type, property.PropertyType},
        source.Expression,
        whereLambda
    );

    return source.Provider.CreateQuery<T>(query);
}

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