How to write string.Contains(someText) in expression Tree

Question

This is the tutorial I\'m following to learn Expression Tree.

I\'ve more than 35 columns to display, but the user can chose to display 10 columns at once. So one the use

Answer 1

You are pretty close, except constructing the call of Contains does not have a right side:

Expression col = Expression.Property(pe, columnName);
Expression contains = Expression.Call(
    pe
,   typeof(string).GetMethod("Contains") // Make a static field out of this
,   Expression.Constant(searchText)      // Prepare a shared object before the loop
);

Once you have your call expressions, combine them with OrElse to produce the body of your lambda. You can do it with loops, or you can use LINQ:

private static readonly MethodInfo Contains = typeof(string).GetMethod(nameof(string.Contains));

public static Expression> SearchPredicate(IEnumerable properties, string searchText) {
    var param = Expression.Parameter(typeof(Student));
    var search = Expression.Constant(searchText);
    var components = properties
        .Select(propName => Expression.Call(Expression.Property(param, propName), Contains, search))
        .Cast()
        .ToList();
    // This is the part that you were missing
    var body = components
        .Skip(1)
        .Aggregate(components[0], Expression.OrElse);
    return Expression.Lambda>(body, param);
}