使用表达式树构建动态选择

Question

如何使用表达式树生成以下内容...

var people = context.Set<Person>();
var transactions = context.Set<FinancialTransaction>();

var dataview = people.Where( p => p.LastName == "Smith" );

var selection = dataview
        .Select( p => new
        {
            FirstName = p.FirstName,
            LastName = p.LastName,
            LastTransaction =
                transactions
                    .Where( t => t.AuthorizedPersonId == p.Id )
                    .Max( t => t.TransactionDateTime )
        } );

gReport.AutoGenerateColumns = true;
gReport.DataSource = selection.ToList();
gReport.DataBind();

我正在尝试使用Ethan Brown 在这里提供的LinqRuntimeTypeBuilder解决方案,但在如何为LastTransaction子查询创建表达式以及如何将查询绑定到GridView方面苦苦挣扎.

这就是我到目前为止......

var people = context.Set<Person>();
var transactions = context.Set<FinancialTransaction>();

var dataview = people.Where( p => p.LastName == "Smith" );

var dynamicFields = new Dictionary<string, Type>();
dynamicFields.Add( "FirstName", typeof( string ) );
dynamicFields.Add( "LastName", typeof( string ) );
dynamicFields.Add( "LastTransaction", typeof( DateTime? ) );

Type dynamicType = Rock.Data.LinqRuntimeTypeBuilder.GetDynamicType( dynamicFields );

ParameterExpression sourceItem = Expression.Parameter( dataview.ElementType, "x" );

// Is this right? if if so how do I bind it to the dynamic field????
Expression<Func<Person, DateTime>> lastTransactionSelect = a => transactions.Where( t => t.AuthorizedPersonId == a.Id && t.TransactionDateTime.HasValue ).Max( t => t.TransactionDateTime.Value );

var bindings = new List<MemberBinding>();
bindings.Add( Expression.Bind( dynamicType.GetField( "FirstName" ), Expression.Property( sourceItem, dataview.ElementType.GetProperty( "FirstName" ) ) ) );
bindings.Add( Expression.Bind( dynamicType.GetField( "LastName" ), Expression.Property( sourceItem, dataview.ElementType.GetProperty( "LastName" ) ) ) );
bindings.Add( Expression.Bind( dynamicType.GetField( "LastTransaction" ), ??? ) );

Expression selector = Expression.Lambda( Expression.MemberInit( Expression.New( dynamicType.GetConstructor( Type.EmptyTypes ) ), bindings ), sourceItem );

var query = dataview.Provider.CreateQuery(
    Expression.Call(
        typeof( Queryable ),
        "Select",
        new Type[] { dataview.ElementType, dynamicType },
    Expression.Constant( dataview ), selector ) ).AsNoTracking();

// Can't bind directly to the query since it's a DBQuery object
gReport.DataSource = ???;

gReport.DataBind();

如何为子查询创建表达式,然后将查询绑定到GridView的最佳方法是什么？

Answer 1

在使用Reflector来评估编译器如何生成linq语句之后,这里是我最终为子选择创建表达式的方法......

ParameterExpression transactionParameter = Expression.Parameter(typeof(FinancialTransaction), "t");
MemberExpression authorizedPersonIdProperty = Expression.Property(transactionParameter, "AuthorizedPersonId");
MemberExpression transactionDateTime = Expression.Property(transactionParameter,"TransactionDateTime");

MethodInfo whereMethod = GetWhereMethod();
MethodInfo maxMethod = GetMaxMethod();

var personIdCompare = new Expression[] { 
    Expression.Constant(transactions), 
    Expression.Lambda<Func<FinancialTransaction, bool>>( Expression.Equal(authorizedPersonIdProperty, Expression.Convert(idProperty, typeof(int?))), new ParameterExpression[] { transactionParameter } ) 
};
var transactionDate = Expression.Lambda<Func<FinancialTransaction, DateTime?>>( transactionDateTime, new ParameterExpression[] { transactionParameter } );
var lastTransactionDate = Expression.Call( null, maxMethod, new Expression[] { Expression.Call( null, whereMethod, personIdCompare ), transactionDate } );

...

bindings.Add( Expression.Bind( dynamicType.GetField( "LastTransaction" ), lastTransactionDate ) );


...


private MethodInfo GetWhereMethod()
{
    Func<FinancialTransaction, bool> fake = element => default( bool );
    Expression<Func<IEnumerable<FinancialTransaction>, IEnumerable<FinancialTransaction>>> lamda = list => list.Where( fake );
    return ( lamda.Body as MethodCallExpression ).Method;
}

private MethodInfo GetMaxMethod()
{
    Func<FinancialTransaction, DateTime?> fake = element => default( DateTime? );
    Expression<Func<IEnumerable<FinancialTransaction>, DateTime?>> lamda = list => list.Max( fake );
    return ( lamda.Body as MethodCallExpression ).Method;
}