How to create an Expression tree to do the same as "StartsWith"
.net c# expression-trees lambda vb.net
Question
Currently, I have this method to compare two numbers
Private Function ETForGreaterThan(ByVal query As IQueryable(Of T), ByVal propertyValue As Object, ByVal propertyInfo As PropertyInfo) As IQueryable(Of T)
Dim e As ParameterExpression = Expression.Parameter(GetType(T), "e")
Dim m As MemberExpression = Expression.MakeMemberAccess(e, propertyInfo)
Dim c As ConstantExpression = Expression.Constant(propertyValue, propertyValue.GetType())
Dim b As BinaryExpression = Expression.GreaterThan(m, c)
Dim lambda As Expression(Of Func(Of T, Boolean)) = Expression.Lambda(Of Func(Of T, Boolean))(b, e)
Return query.Where(lambda)
End Function
It works fine and is consumed in this way
query = ETForGreaterThan(query, Value, propertyInfo)
As you can see, I give it an IQueryable collection and it add a where clause to it, base on a property and a value. Y can construct Lessthan, LessOrEqualThan etc equivalents as System.Linq.Expressions.Expression has this operators predefined.
¿How can I transform this code to do the same with strings? System.Linq.Expressions.Expression don't give me a predefined operator like "contains" or "startwith" and I'm really noob with Expression trees.
Thanks, and please Post your answer in C#/VB. Choose the one that make you feel more confortable.
Accepted Answer
using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
namespace WindowsFormsApplication1
{
static class Program
{
[STAThread]
static void Main()
{
using (var context = new NorthwindEntities())
{
PropertyInfo propertyInfo = typeof(Customer).GetProperty("CustomerID");
IQueryable<Customer> query = context.Customers;
query = ETForStartsWith<Customer>(query, "A", propertyInfo);
var list = query.ToList();
}
}
static IQueryable<T> ETForStartsWith<T>(IQueryable<T> query, string propertyValue, PropertyInfo propertyInfo)
{
ParameterExpression e = Expression.Parameter(typeof(T), "e");
MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
ConstantExpression c = Expression.Constant(propertyValue, typeof(string));
MethodInfo mi = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
Expression call = Expression.Call(m, mi, c);
Expression<Func<T, bool>> lambda = Expression.Lambda<Func<T, bool>>(call, e);
return query.Where(lambda);
}
}
}
Popular Answer
It's not an operator, but a method, so you can call it with Expression.Call(), where the methodinfo parameter will be typeof(string).GetMethod("StartsWith").