Question
I'm studying "Expression Tree" but I'm not managing to perform these expressions:
// first case
someList.Select(p => p.SomeProperty);
and
// second case
someList.Select(p => new OtherClass
{
SomeProperty = p.SomeProperty
})
To the "first case" I tried do this:
var someList = new List<SomeClass>();
someList.Add(new SomeClass { SomeProperty = "Hello" });
var someParam = Expression.Parameter(typeof(SomeClass), "p");
var someProperty = Expression.Property(someParam, "SomeProperty");
Expression.Call(
typeof(Enumerable),
"Select",
new Type[]
{
typeof(SomeClass),
typeof(string)
},
Expression.Lambda(
someProperty,
someParam
)
).Dump();
But I get this error:
InvalidOperationException: No generic method 'Select' on type 'System.Linq.Enumerable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.
About the "second case", I don't have ideia how to proceed.
Can anyone guide me here?
Accepted Answer
Calm down folks, after some research I found what was missing in my code...
On the fist case:
Expression.Call(
typeof(Enumerable),
"Select",
new Type[]
{
typeof(SomeClass),
typeof(string)
},
Expression.Constant(someList), // <---------------- HERE IT IS
Expression.Lambda(
someProperty,
someParam
)
);
To the second case, I created the "new" expression through the code below:
var bind = Expression.Bind(typeof(OtherClass).GetProperty("SomeProperty"), someProperty);
var otherClassNew = Expression.New(typeof(OtherClass));
var otherClassInit = Expression.MemberInit(otherClassNew, bind);
Anyway, Thank you all for your help!
Popular Answer
Some examples of what you could do:
Given
public class SomeClass
{
public string SomeProperty { get; set; }
}
and
var someList = new List<SomeClass>();
someList.Add(new SomeClass { SomeProperty = "Hello" });
var someParam = Expression.Parameter(typeof(SomeClass), "p");
var someProperty = Expression.Property(someParam, "SomeProperty");
Expression<Func<SomeClass, string>> lambda = Expression.Lambda<Func<SomeClass, string>>(someProperty, someParam); // p => p.SomeProperty
Using an IEnumerable<SomeClass>... Note the .Compile()
Func<SomeClass, string> compiled = lambda.Compile();
IEnumerable<string> q1 = someList.Select(compiled);
You shouldn't ever use AsQueryable() but in unit tests and "experimentation" programs (like this one). Just to make @Peter happy, I'll add another possible condition: if you really know what it does (not think you know what it does, really!), then you can use it. But if it the first time you use it, I still suggest you ask on SO if you are right in using it.
IQueryable<SomeClass> queryable = someList.AsQueryable();
Directly using the Queryable.Select()
IQueryable<string> q2 = queryable.Select(lambda);
Building a Select and using the CreateQuery (this is very similar to what internally the Queryable.Select does) to "inject" it in the query.
MethodInfo select = (from x in typeof(Queryable).GetMethods()
where x.Name == "Select" && x.IsGenericMethod
let gens = x.GetGenericArguments()
where gens.Length == 2
let pars = x.GetParameters()
where pars.Length == 2 &&
pars[0].ParameterType == typeof(IQueryable<>).MakeGenericType(gens[0]) &&
pars[1].ParameterType == typeof(Expression<>).MakeGenericType(typeof(Func<,>).MakeGenericType(gens))
select x).Single().MakeGenericMethod(typeof(SomeClass), typeof(string));
MethodCallExpression select2 = Expression.Call(null, select, Expression.Constant(queryable), lambda);
IQueryProvider provider = queryable.Provider;
IQueryable<string> q3 = provider.CreateQuery<string>(select2);