Convert func to predicate using reflection in C #

Question

I mainly try to make this one , but I don’t know what T is, so I build things using reflection and expression trees.

// Input (I don't know about "Book")
Type itemType = typeof(Book);

// Actual Code
// Build up func p => p.AuthorName == "Jon Skeet"
ParameterExpression predParam = Expression.Parameter(itemType, "p");
Expression left = Expression.Field(predParam, itemType.GetField("AuthorName"));
Expression right = Expression.Constant("Jon Skeet", typeof(string));
Expression equality = Expression.Equal(left, right);
Delegate myDelegate = Expression.Lambda(equality, new ParameterExpression[] { predParam }).Compile(); // Not sure if I need this

// Build up predicate type (Predicate<Book>)
Type genericPredicateType = typeof(Predicate<>);
Type constructedPredicateType = genericPredicateType.MakeGenericType(new Type[] { itemType });

// I need an instance to use this predicate, right? (** This Fails **)
object predicateInstance = Activator.CreateInstance(constructedPredicateType, new object[] { myDelegate });

Basically, I have List<Book>, which I am trying to reflect, and Invokeits method Find. The method Findis required Predicate<Book>instead Func<Book, bool>, and I hit my head about it several times.

Edit: here is the first part of the error trace:

System.MissingMethodException: Constructor on type 'System.Predicate`1[[MyProject.Book, MyProject, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]' not found.

Answer 1

Fortunately, this is fairly easy to do by simply changing your call to Expression.Lambda:

Type predicateType = typeof(Predicate<>).MakeGenericType(itemType);
LambdaExpression lambda = Expression.Lambda(predicateType, equality, predParam);
Delegate compiled = lambda.Compile();

, , ... , .

Answer 2

, , :

public static Predicate<T> GetPredicate<T>()
{
    Type itemType = typeof(T);
    ParameterExpression predParam = Expression.Parameter(itemType, "p");
    Expression left = Expression.Field(predParam, itemType.GetField("AuthorName"));
    Expression right = Expression.Constant("Jon Skeet", typeof(string));
    Expression equality = Expression.Equal(left, right);
    Func<T, bool> function = (Func<T, bool>)Expression.Lambda(equality, new[] { predParam }).Compile();
    return new Predicate<T>(function);
}