Expression predicates with field name as parameter

Question

I use this piece of code (found on stackoverflow) to generate a predicate

static class BuilderPredicate
{
    public static Expression<Func<T, bool>> True<T>() { return f => true; }
    public static Expression<Func<T, bool>> False<T>() { return f => false; }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
                                                        Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>
              (Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
                                                         Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>
              (Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
    }
}

I have this object :

public class Person : IPerson
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public interface IPerson
{
    int Id { get; set; }
    string FirstName { get; set; }
    string LastName { get; set; }
}

To use to, I do this :

private void CreationPredicate(string fieldname, string stringToSearch)
{
    var predicate = BuilderPredicate.True<Person>();
    switch (fieldname)
    {
        case "FirstName":
            predicate = predicate.And(e => e.FirstName.StartsWith(stringToSearch));
            break;
        case "LastName":
            predicate = predicate.And(e => e.LastName.StartsWith(stringToSearch));
            break;
    }
}

I'd like avoid, the switch and replace e => e.FirstName.StartWith by (if possible)

e => e.fieldname.StartWith

How can I do this ?

Thanks,

Answer 1

If you are using strings, you need to build the expression the hard way:

var param = Expression.Parameter(typeof (Foo));
var pred = Expression.Lambda<Func<Foo, bool>>(
    Expression.Call(
        Expression.PropertyOrField(param, fieldName),
        "StartsWith",null,
        Expression.Constant(stringToSearch)), param);

On 4.0, I'd also have used an ExpressionVisitor to rewrite the body for the "and", rather than an Invoke; Invoke is not supported on EF etc.