How do I dynamically create an Expression<Func<MyClass, bool>> predicate from Expression<Func<MyClass, string>>?
I trying to append where predicates and my goal is to create the same expression as:
Services.Where(s => s.Name == "Modules" && s.Namespace == "Namespace");
I have the following code:
Expression<Func<Service,string>> sel1 = s => s.Name;
Expression<Func<Service,string>> sel2 = s => s.Namespace;
var val1 = Expression.Constant("Modules");
var val2 = Expression.Constant("Namespace");
Expression e1 = Expression.Equal(sel1.Body, val1);
Expression e2 = Expression.Equal(sel2.Body, val2);
var andExp = Expression.AndAlso(e1, e2);
ParameterExpression argParam = Expression.Parameter(typeof(string), "s");
var lambda = Expression.Lambda<Func<string, bool>>(andExp, argParam);
This create the following output:
s => ((s.Name == "Modules") AndAlso (s.Namespace == "Namespace"))
However, this is faulty since the parameter for Name and Namespace isn't the same. If I change one of the expression selector to:
Expression<Func<Service,string>> sel2 = srv => srv.Namespace;
The output will be:
s => ((s.Name == "Modules") AndAlso (srv.Namespace == "Namespace"))
How can I create a valid expression with use of sel1 and sel2?
UPDATE (28 feb 2011)
I solved it by creating invoke expressions: Expression.Invoke
so the lambda expressions sel1 and sel2 don't necessary need to be a MemberExpression:
Expression<Func<Service,string>> sel1 = s => s.Name;
Expression<Func<Service,string>> sel2 = srv => srv.Namespace;
var val1 = Expression.Constant("Modules");
var val2 = Expression.Constant("Namespace");
Expression<Func<Service, bool>> lambda = m => true;
var modelParameter = lambda.Parameters.First();
// sel1 predicate
{
var invokedExpr = Expression.Invoke(sel1, modelParameter);
var binaryExpression = Expression.Equal(invokedExpr, val1);
lambda = Expression.Lambda<Func<Service, bool>>(Expression.AndAlso(binaryExpression, lambda.Body), lambda.Parameters);
}
// sel2 predicate
{
var invokedExpr = Expression.Invoke(sel2, modelParameter);
var binaryExpression = Expression.Equal(invokedExpr, val2);
lambda = Expression.Lambda<Func<Service, bool>>(Expression.AndAlso(binaryExpression, lambda.Body), lambda.Parameters);
}
Solution 1:
It's hard to mix compiler-generated expression trees and hand-made ones, precisely because of this sort of thing - extracting out the ParameterExpressions is tricky. So let's start from scratch:
ParameterExpression argParam = Expression.Parameter(typeof(Service), "s");
Expression nameProperty = Expression.Property(argParam, "Name");
Expression namespaceProperty = Expression.Property(argParam, "Namespace");
var val1 = Expression.Constant("Modules");
var val2 = Expression.Constant("Namespace");
Expression e1 = Expression.Equal(nameProperty, val1);
Expression e2 = Expression.Equal(namespaceProperty, val2);
var andExp = Expression.AndAlso(e1, e2);
var lambda = Expression.Lambda<Func<Service, bool>>(andExp, argParam);
One important aspect I've changed is the type passed to Expression.Parameter
- it certainly looks like it should be a Service
rather than a string
.
I've given that a try, and it seemed to work when I called lambda.Compile
and executed it on a couple of sample Service
objects...
Solution 2:
You can create an Expression tree for nullable types, suppose you have a nullable field BoardId, you can create expression tree dynamically like this
var nameValue="BoardId=111";
you need to determine first Property type, whether its Nullable or not
Below code create a Dynamic tree expression for nullable and Non Nullable types
public static Expression<Func<T, bool>> BuildWhereExpression<T>(string nameValueQuery ) where T : class
{
Expression<Func<T, bool>> predicate = null;
PropertyInfo prop = null;
var fieldName = nameValueQuery.Split("=")[0];
var fieldValue = nameValueQuery.Split("=")[1];
var properties = typeof(T).GetProperties();
foreach (var property in properties)
{
if (property.Name.ToLower() == fieldName.ToLower())
{
prop = property;
}
}
if (prop != null)
{
var isNullable = prop.PropertyType.IsNullableType();
var parameter = Expression.Parameter(typeof(T), "x");
var member = Expression.Property(parameter, fieldName);
if (isNullable)
{
var filter1 =
Expression.Constant(
Convert.ChangeType(fieldValue, member.Type.GetGenericArguments()[0]));
Expression typeFilter = Expression.Convert(filter1, member.Type);
var body = Expression.Equal(member, typeFilter);
predicate = Expression.Lambda<Func<T, bool>>(body, parameter);
}
else
{
if (prop.PropertyType == typeof(string) && likeOerator.ToLower() == "like")
{
var parameterExp = Expression.Parameter(typeof(T), "type");
var propertyExp = Expression.Property(parameterExp, prop);
MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
var someValue = Expression.Constant(fieldValue, typeof(string));
var containsMethodExp = Expression.Call(propertyExp, method, someValue);
predicate = Expression.Lambda<Func<T, bool>>(containsMethodExp, parameterExp);
}
else
{
var constant = Expression.Constant(Convert.ChangeType(fieldValue, prop.PropertyType));
var body = Expression.Equal(member, constant);
predicate = Expression.Lambda<Func<T, bool>>(body, parameter); `enter code here`
}
}
}
return predicate;
}
1- This Solution first checks for the Nullable value and generate the expression. This is How you can determine if the type is Nullable. I have created an extension method for that purpose
public static bool IsNullableType(this Type type) { return
type.IsGenericType &&
(type.GetGenericTypeDefinition().Equals(typeof(Nullable<>))); }
2- the second step is to check the type if its string then create an expression for a string.
3- the Third step is to check is value is not nullable not string then create an expression using equal