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How do I find out the type specified by an Expression?

Answer 1

A:

Richard 2010-08-12 15:58:37

@Richard: I'm using this to define fetching strategies for a repository, so the "path" expression would be something like entity => entity.Property; I need to determine if that "Property" is a collection type. I hope this helps to clarify.

DanP 2010-08-12 16:05:12

@DanP: The `Type` property gives you a `Type` instance, you can then check for implementation of `IEnumerable` (without which a type in .NET isn't really a collection) in the normal way.

Richard 2010-08-12 17:27:16

@Richard: this will give me the type of the root object that owns the property; I was interested in the type of the property itself.

DanP 2010-08-12 19:11:43

@DanP: You need to look at the `Expression` subtype, not all represent a property (look at the example in the Expression.Type doc -- linked in the A).

Richard 2010-08-12 20:48:05

Answer 2

+2 A:

You're already using generics, so why not go all the way?

void GetMemberType<TArg, TProperty>(Expression<Func<TArg, TProperty>> path)
{
    // Get the member ...
    var member = path.Body as MemberExpression;

    // ... Do whatever you want with the member ...

    // Now get the type
    var memberType = typeof(TProperty);
}

Charles 2010-08-12 16:31:05

Type inference will let you omit the two generic type params when you call the method, assuming TArg happens to be a generic type param used by the declaring class - as in `class SomeClass<TArg> { ... }`.

Charles 2010-08-12 16:35:59

typeof(IEnumerable).IsAssignableFrom(typeof(TProperty)) gets DanP the rest of the way there.

Kirk Woll 2010-08-12 16:37:12

Thanks to Carles and Kirk - this is exactly what I *should* have been doing :)

DanP 2010-08-12 16:50:44

I added an alternative method as another answer - and it matches DanP's `Expression<Func<T, object>> path`, as opposed to my more generic `Expression<Func<TArg, TProperty>> path`.

Charles 2010-08-12 16:51:27

Answer 3

+1 A:

Alternatively, you could do something like the following (which is more inline with you generic type arguments):

// Get your expression
// (probably already provided as an arg to one of your methods, I'm willing to bet)
Expression<Func<T, object>> path = ...

var memberExpr = path.Body as MemberExpression;
var member = memberExpr.Member;

Type type;

if (member is FieldInfo)
{
    var field = member as FieldInfo;
    type = field.FieldType;
}

if (member is PropertyInfo)
{
    var property = member as PropertyInfo;
    type = property.PropertyType;
}

Charles 2010-08-12 16:46:22

@Charles: thanks for adding this alternative method, good to know if I couldn't use the infered generics for some reason.

DanP 2010-08-12 19:10:05

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How do I find out the type specified by an Expression?

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