Dynamic lambda expression for array property filter

Question

As per requirement I want to create a dynamic lambda expression using C#.

For example I want to generate the dynamic query like

Employee. Address[1].City

How can I do this? Please note that the property is a dynamic one.

I have tried this code

var item = Expression.Parameter(typeof(Employee), "item");

Expression prop = Expression.Property(item, "Address", new Expression[] { Expression.Constant[1] });
prop = Expression.Property(prop, "City");

var propValue = Expression.Constant(constraintItem.State);
var expression = Expression.Equal(prop, propValue);
var lambda = Expression.Lambda<Func<Line, bool>>(expression, item);

But it did not work.

Any help would be appreciated.

Thanks.

Is Address an indexer? If it's an array, use Expression.ArrayIndex, If it's just a list with an indexer, retrieve the list using an unindexed Expression.Property, and then apply an indexed property retrieval expression on it's Items property. — Lucas Trzesniewski
– Lucas Trzesniewski, Commented Dec 6, 2015 at 13:47

Kirill Shlenskiy · Accepted Answer · 2015-12-06 15:06:36Z

3

You "dynamic query" expression (which is not really a query, it's a simple MemberExpression) can be produced as follows:

ParameterExpression param = Expression.Parameter(typeof(Employee), "item");
MemberExpression address = Expression.Property(param, "Address");
BinaryExpression indexedAddress = Expression.ArrayIndex(address, Expression.Constant(1));
MemberExpression city = Expression.Property(indexedAddress, "City"); // Assuming "City" is a string.

// This will give us: item => item.Address[1].City
Expression<Func<Employee, string>> memberAccessLambda = Expression.Lambda<Func<Employee, string>>(city, param);

If you want an actual predicate to use as part of your query, you just wrap the MemberExpression with a relevant compare expression, i.e.

BinaryExpression eq = Expression.Equal(city, Expression.Constant("New York"));

// This will give us: item => item.Address[1].City == "New York"
Expression<Func<Employee, bool>> predicateLambda = Expression.Lambda<Func<Employee, bool>>(eq, param);

In terms of your code: not sure why you're creating a lambda where the delegate type is Func<Line, bool> when the input is clearly expected to be Employee. Parameter type must always match the delegate signature.

EDIT

Non-array indexer access example:

ParameterExpression param = Expression.Parameter(typeof(Employee), "item");
MemberExpression address = Expression.Property(param, "Address");

IndexExpression indexedAddress = Expression.MakeIndex(
    address,
    indexer: typeof(List<string>).GetProperty("Item", returnType: typeof(string), types: new[] { typeof(int) }),
    arguments: new[] { Expression.Constant(1) }
);

// Produces item => item.Address[1].
Expression<Func<Employee, string>> lambda = Expression.Lambda<Func<Employee, string>>(indexedAddress, param);

// Predicate (item => item.Address[1] == "Place"):
BinaryExpression eq = Expression.Equal(indexedAddress, Expression.Constant("Place"));
Expression<Func<Employee, bool>> predicateLambda = Expression.Lambda<Func<Employee, bool>>(eq, param);

edited Dec 6, 2015 at 15:06

answered Dec 6, 2015 at 14:11

Kirill Shlenskiy

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6 Comments

Ranish Over a year ago

Thanks for your reply. A slight change in requirement. Address is a List<String> property in Employee class.

Ranish Over a year ago

I am getting following error "An unhandled exception of type 'System.ArgumentException' occurred in System.Core.dll"

Ranish Over a year ago

requirement is generate the query for x.Address[1]=='Place'. Thanks!!

Kirill Shlenskiy Over a year ago

That's because ArrayIndex only works on arrays (not lists). I'll amend my code and I hope you'll be able to take it from there (at this point it is not really clear what kind of predicate you will be producing using your List<string>).

Kirill Shlenskiy Over a year ago

Thanks for clarifying. Amended.

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