Question

I’ve got a bit of a challenge where I have to create an expression tree to represent a query input by the user. Since I don’t have the time to create all the possible cases of user input, I figured that expression trees would aid me in solving this.

For the most part, it has. I am, however, a bit stumped. I am in the code below trying to perform a List.Find with a dynamically created expression. The expression, in short, is this:

list.Find(m => m.ListOfStrings.Exists(s => s == "cookie"));

where m is

class MyClass
{
    public List<string> ListOfStrings { get; set; }
}

I’ve gotten so far as to create

s => s == "cookie"

with expressions, no problem. I’ve also declared a methodinfo for Exists

var existsMethod = typeof(MyClass)
        .GetProperty("ListOfStrings")
        .PropertyType
        .GetMethod("Exists");

The only problem I have is creating an expression to invoke said method with the lambda as a parameter like so

var findLambda = Expression.Lambda(
    Expression.Call(
        Expression.Property(
            Expression.Parameter(typeof(MyClass), "m"),
            typeof(MyClass).GetProperty("ListOfStrings")),
        existsMethod,
        existsLambda),
    Expression.Parameter(
        typeof (MyClass),
        "m"));

It gives an understandable exception that

Expression of type 'System.Func`2[System.String,System.Boolean]' cannot be used for parameter of type 'System.Predicate`1[System.String]' of method 'Boolean Exists(System.Predicate`1[System.String])'

How the heck can I overcome this?

Full code:

private class MyClass
{
    public List<string> ListOfStrings { get; set; }
}

public void SomeMethod()
{
    var myObject = new MyClass();
    myObject.ListOfStrings = new List<string>();
    myObject.ListOfStrings.Add("cookie");
    myObject.ListOfStrings.Add("biscuit");

    List<MyClass> list = new List<MyClass>();
    list.Add(myObject);

    var existsLambda = Expression.Lambda(
        Expression.Equal(
            Expression.Parameter(typeof(string), "s"),
            Expression.Constant("cookie")),
        Expression.Parameter(typeof(string), "s"));

    var existsMethod = typeof(MyClass).GetProperty("ListOfStrings").PropertyType.GetMethod("Exists");

    var findLambda = Expression.Lambda(
        Expression.Call(
            Expression.Property(
                Expression.Parameter(typeof(MyClass), "m"),
                typeof(MyClass).GetProperty("ListOfStrings")),
            existsMethod,
            existsLambda),
        Expression.Parameter(
            typeof (MyClass),
            "m"));

    list.Find((Predicate<MyClass>)findLambda.Compile());
}

Answer 1

The delegates have different types:

public delegate bool Predicate<T>(T obj);
public delegate TResult Func<T, TResult>(T arg);

The Exists method (and the Find) expect Predicate<T>. The Lambda expression compiles at runtime to Func<T, TResult>.

Try the following:

var existsLambda = Expression.Lambda(typeof(Predicate<string>), 
                Expression.Equal(
                    Expression.Parameter(typeof(string), "s"),
                    Expression.Constant("cookie")),
                Expression.Parameter(typeof(string), "s"));

You can also use the generic Lambda Function:

var existsLambda = Expression.Lambda<Predicate<string>>(Expression.Equal(
                    Expression.Parameter(typeof(string), "s"),
                    Expression.Constant("cookie")),
                Expression.Parameter(typeof(string), "s"));