Linq表达式 - >构建一个位置（...）。单个（...）Linq树表达式

Question

I'm trying to build an Expression<Func<TEntity, TKey>> like:

e.Collection.Where(c => c.Key.Equals("key")).Single()

So, up to now, I've been able to build something like that. However, I'm not quite to figure out how to build Where().Single() chain:

Type entityType = typeof(TElementType);
PropertyInfo collectionPropertyInfo = entityType.GetProperty("Metainfos"); // TODO: Pick the property up instead of using a literal string
if (collectionPropertyInfo == null)
    throw new MissingFieldException(string.Format("{0} collection doesn't appear in {1}", "MetaInfos", entityType));

Type collGenericType = collectionPropertyInfo.PropertyType.GetGenericArguments().FirstOrDefault();
if (!collGenericType.IsAssignableFrom(typeof(Domain.MetaInfoValue)))
    throw new TypeLoadException(string.Format("Collection generic type doesn't inherit from {1}", collGenericType));

ParameterExpression entityParameter = Expression.Parameter(entityType, "t");
ParameterExpression metaInfoParameterExpression = Expression.Parameter(collGenericType, "m");

MemberExpression collectionMemberExpression = Expression.Property(entityParameter, collectionPropertyInfo);
MethodInfo whereMethod = typeof(Enumerable).GetMethods().Where(m => m.Name.Equals("Where") && m.GetParameters().Length == 2).First().MakeGenericMethod(collGenericType);
MethodInfo singleMethod = typeof(Enumerable).GetMethods().Where(m => m.Name.Equals("Single") && m.GetParameters().Length == 1).First().MakeGenericMethod(collGenericType);

LambdaExpression innerCondition = Expression.Lambda(
    Expression.GetDelegateType(collGenericType, typeof(bool)),
    Expression.Equal(
        Expression.Property(metaInfoParameterExpression, "Key"),
        Expression.Constant(field)
    ),
    metaInfoParameterExpression
);

return Expression.Lambda<Func<TElementType, TKeyType>>(
    Expression.Call(
        singleMethod,
        Expression.Lambda<Func<TElementType, bool>>(
            Expression.Call(whereMethod, collectionMemberExpression, innerCondition), 
            entityParameter
        )
    )
);}

It throws me an ArgumentException :

It's not allowed to use an expression of type System.Collections.Generic.IEnumerable1[Backend.Domain.MetaInfoValue]</code> for the returned value System.Boolean`

What's wrong?

Answer 1

It seems like the Expression.Call for Single is the problem. You need to provide (1) the source and (2) the arguments . Right now, you seem to be trying to provide both of them as the same argument ( Expression.Lambda<Func<TElementType, bool>>(Expression.Call(whereMethod, collectionMemberExpression, innerCondition), entityParameter); ).

Think what a Single expression usually looks like: myEnumerable.Single(o => o.Key == iKey); . See how your Source ( myEnumerable ) is different than your selection expression ( o => o.Key == iKey )?

If you create two expressions as arguments for your Single call (one source with Enumerable input and output, and one selector ( Expression.Lambda<Func<TElementType, bool>> ), it should solve the issue.