Objective: process an object and if the object implements an expected type, I want to change a specific property value (this part is working fine), and I also would like to apply the same logic to all property lists (that I explicit point) that are of the same expected type.
I have the following code:
public abstract class BaseObject
{
public int Id { get; set; }
}
public class Father : BaseObject
{
public DateTime CreatedOn { get; set; }
public string Name { get; set; }
public IEnumerable<ChildA> Children1 { get; set; }
public IEnumerable<ChildB> Children2 { get; set; }
public IEnumerable<ChildA> Children3 { get; set; }
public IEnumerable<ChildB> Children4 { get; set; }
}
public class ChildA : BaseObject
{
public int Val1 { get; set; }
}
public class ChildB : BaseObject
{
public string Name { get; set; }
public int Total { get; set; }
}
I want to process an object by applying some changes on a specific property on the target object and on all property children that I explicit say:
public void Start()
{
var listA = new List<ChildA> { new ChildA { Id = 1, Val1 = 1 }, new ChildA { Id = 2, Val1 = 2 } };
var listB = new List<ChildB> { new ChildB { Id = 1, Name = "1", Total = 1 } };
var obj = new Father { Id = 1, CreatedOn = DateTime.Now, Name = "F1", ChildrenA = listA, ChildrenB = listB };
// I explicit tell to process only 2 of the 4 lists....
ProcessObj(obj, x => new object[] { x.Children1, x.Children2 });
}
I was able to write this function:
public void ProcessObj<T>(T obj, Expression<Func<T, object[]>> includes = null)
{
var objBaseObject = obj as BaseObject;
if (objBaseObject == null) return;
// Here I change the ID - add 100 just as an example....
objBaseObject.Id = objBaseObject.Id + 100;
if (includes == null) return;
var array = includes.Body as NewArrayExpression;
if (array == null) return;
var exps = ((IEnumerable<object>)array.Expressions).ToArray();
for (var i = 0; i < exps.Count(); i++)
{
var name = ((MemberExpression)exps[i]).Member.Name;
var childProperty = obj.GetType().GetProperties(
BindingFlags.Public | BindingFlags.Instance
).FirstOrDefault(prop => prop.Name == name);
if (childProperty == null) continue;
// NOT correct because I think I am getting a copy of the object
// and not pointing to the object in memory (by reference)
var childList = childProperty.GetValue(obj);
// TODO: loop on the list and apply the same logic as the father....
// change the ID field....
}
}
In this prototype I started writing reflection, but I really would like to avoid it if possible....
How can I do this???
Maybe I'm missing something, but it seems like you're complicating the problem by using expression trees. Can you just not use a regular Action and Func delegates to do this? Why do they need to be expression trees? Here's an example just using delegates:
public void ProcessObj<T>(T obj, Func<T, IEnumerable<object>> includes) {
var objBaseObject = obj as BaseObject;
if (objBaseObject == null) return;
// Create a reusable action to use on both the parent and the children
Action<BaseObject> action = x => x.Id += 100;
// Run the action against the root object
action(objBaseObject);
// Get the includes by just invoking the delegate. No need for trees.
var includes = includes(obj);
// Loop over each item in each collection. If the types then invoke the same action that we used on the root.
foreach(IEnumerable<object> include in includes)
{
foreach(object item in include)
{
var childBaseObject = item as BaseObject;
if(childBaseObject != null)
{
action(childBaseObject);
}
}
}
}
Useable just like before:
ProcessObj(obj, x => new object[] { x.Children1, x.Children2 });
No expression trees and no reflection, just regular delegate lambdas.
Hope that helps
Related
I have a method which looks like
private Component[] AssociateComponentsWithParametersAndValues(
IEnumerable<Component> components,
IEnumerable<ComponentParameter> parameters,
IEnumerable<ComponentParameterValue> values
)
{
var componentsDictionary = new Dictionary<string, Component>();
var parametersDictionary = new Dictionary<string, ComponentParameter>();
var valuesDictionary = new Dictionary<string, ComponentParameterValue>();
foreach (Component c in components)
{
bool componentMatch = components.Any(co => co == c);
bool identifierMatch = components.Any(co => co.Identifier == c.Identifier);
if (!componentsDictionary.ContainsKey(c.Identifier))
componentsDictionary.Add(c.Identifier, c);
}
// Do a bunch of stuff to mutate the components
return components.ToArray();
}
You would think that componentMatch and identifierMatch would both be true each time right? Instead, componentMatch is always false and identifierMatch is always true. Also, the identifier is (nearly, occasionally there's some bad data) always unique, so it's not like it can be finding another component with the same identifier.
So, there must be something weird with the Component class. Well, here's what it looks like
public class Component : ConfigurationObject
{
public string Parent { get; set; }
public string Type { get; set; }
public string Module { get; set; }
public string Code { get; set; }
public string Name { get; set; }
public string Description { get; set; }
public string TypeName { get; set; }
public bool? Enabled { get; set; }
public string DBIdentifier { get; set; }
public Dictionary<string, ComponentParameterAndValues> ParametersAndValues { get; set; }
public override string Identifier => DBIdentifier;
}
And here's the class it implements
public abstract class ConfigurationObject
{
public abstract string Identifier { get; }
}
Why is this happening?
The only way I could see this break, is if IEnumerable<Component> components is a lazily evaluated enumerable, returning new iterator objects every time. This works:
var list = new List<Component>
{
new Component { Identifier = "Foo" },
new Component { Identifier = "Bar" },
new Component { Identifier = "Baz" },
};
foreach (Component c in list)
{
bool componentMatch = list.Any(co => co == c);
Console.WriteLine($"Component {c.Identifier} match: {componentMatch}");
}
Because == checks reference equality (unless Component overrides it, but it doesn't look like it does). However, if it's not a list, but a new result each iteration:
IEnumerable<Component> list = GetList();
foreach (Component c in list)
{
bool componentMatch = list.Any(co => co == c);
Console.WriteLine($"Component {c.Identifier} match: {componentMatch}");
}
private static IEnumerable<Component> GetList()
{
yield return new Component { Identifier = "Foo" };
yield return new Component { Identifier = "Bar" };
yield return new Component { Identifier = "Baz" };
}
Then it prints false, because foreach() and Any() each get a new collection of new objects, so their references don't match.
The solution would be to enumerate once, and store the components once, materialized in a list, and use that:
var localComponents = components.ToList();
foreach (Component c in localComponents)
{
// ...
}
Sorry if this is asked and answered, I searched but think I don't know the vocabulary to find the answer. Researched reflection but that doesn't seem to be the answer here? I'm a novice obviously. I'm trying/making minor contributions to a mod for the new Battletech game.
I've got this Dictionary and would like to use its keys to set properties as in the foreach below. I don't know if this is at compile or runtime, my guess is compile time...
I put *limb* in as pseudo-code for how I'm imagining it might work. The property mechDef.Head is an object of type LocationLoadoutDef with its property CurrentInternalStructure being float.
Hope that makes sense!
Much obliged for any assistance.
public class Settings {
public readonly Dictionary<string, bool> LimbRepair = new Dictionary<string, bool> {
{ "Head", false },
{ "LeftArm", false },
{ "RightArm", false },
{ "CenterTorso", false },
{ "LeftTorso", false },
{ "RightTorso", false },
{ "LeftLeg", false },
{ "RightLeg", false },
};
}
MechDef mechDef = new MechDef
(__instance.DataManager.MechDefs.Get(id), __instance.GenerateSimGameUID());
foreach (string limb in settings.LimbRepair.Keys) {
if (!settings.LimbRepair[limb]) {
mechDef.*limb*.CurrentInternalStructure = Math.Max
(1f, mechDef.*limb*.CurrentInternalStructure * (float)rng.NextDouble());
}
You can do it with Reflection, but....
This is quite easy to do with Reflection, and you'll probably get a couple answers on here that show you how, but since you are writing a game, I'm guessing you want the best performance possible, and Reflection isn't always going to give you that.
Below is a solution that requires no reflection but still allows you to use the loop structure you want. It just requires a little bit of setup when you create the object, then you can access your properties as if they were in a dictionary.
Solution: Use a dictionary of delegates to map the properties
First we need to write a utility class that represents a property. Since properties can be different types, this is a generic class with a type argument.
class PropertyWrapper<T>
{
private readonly Func<T> _getter;
private readonly Action<T> _setter;
public PropertyWrapper(Func<T> getter, Action<T> setter)
{
_getter = getter;
_setter = setter;
}
public T Value
{
get
{
return _getter();
}
set
{
_setter(value);
}
}
}
The idea behind this class is that you create it to represent any property you want, and call its methods to read and set the property. The class knows how to read and set the property because you tell it how, when you construct it, by passing it a short lambda expression that does the work.
This utility will allow you to put all the properties that represent limbs into a dictionary. Then you can look them up by string, just like your settings. So for example your MechDefinition might look like this:
class MechDef
{
public Limb Head { get; set; }
public Limb LeftArm { get; set; }
public Limb RightArm { get; set; }
public Limb LeftTorso { get; set; }
public Limb RightTorso { get; set; }
public Limb CenterTorso { get; set; }
public Limb RightLeg { get; set; }
public Limb LeftLeg { get; set; }
private readonly Dictionary<string, PropertyWrapper<Limb>> Properties;
public MechDef()
{
Properties = new Dictionary<string, PropertyWrapper<Limb>>
{
{"Head", new PropertyWrapper<Limb>( () => Head, v => Head = v ) },
{"LeftArm", new PropertyWrapper<Limb>( () => LeftArm, v => LeftArm = v ) },
{"RightArm", new PropertyWrapper<Limb>( () => RightArm, v => RightArm = v ) },
{"CenterTorso",new PropertyWrapper<Limb>( () => CenterTorso, v => CenterTorso = v )},
{"RightTorso", new PropertyWrapper<Limb>( () => RightTorso, v => RightTorso = v ) },
{"LeftTorso", new PropertyWrapper<Limb>( () => LeftTorso, v => LeftTorso = v ) },
{"RightLeg", new PropertyWrapper<Limb>( () => RightLeg, v => RightLeg = v ) },
{"LeftLeg", new PropertyWrapper<Limb>( () => LeftLeg, v => LeftLeg = v ) }
};
foreach (var property in Properties.Values) property.Value = new Limb();
}
public Limb this[string name]
{
get
{
return Properties[name].Value;
}
set
{
Properties[name].Value = value;
}
}
}
Yes, there is a bit of setup there, but it's all in one place, and it only executes once, when you instantiate the MechDef. Now you can access all of the limbs by string:
foreach (var pair in settings.LimbRepair)
{
if (pair.Value != false) continue;
var limb = mechDef[pair.Key];
limb.CurrentInternalStructure = Math.Max
(
1.0F,
limb.CurrentInternalStructure * (float)rng.NextDouble()
);
}
Link to DotNetFiddle example
You can create a DynamicObject to create your own dynamic Dictionary, See the explanation here
Assume that you want to provide alternative syntax for accessing
values in a dictionary, so that instead of writing
sampleDictionary["Text"] = "Sample text", you can write
sampleDictionary.Text = "Sample text".
This is the example from the same MSDN article above:
public class DynamicDictionary : DynamicObject
{
// The inner dictionary
Dictionary<string, object> dictionary = new Dictionary<string, object>();
public int Count
{
get { return dictionary.Count; }
}
// If you try to get a value of a property not defined
// in the class, this method is called.
public override bool TryGetMember(GetMemberBinder binder, out object result)
{
// Converting the property name to lowercase so
// that property names become case-insensitive.
string name = binder.Name.ToLower();
// If the property name is found in a dictionary, set the result parameter
// to the property value and return true. Otherwise, return false.
return dictionary.TryGetValue(name, out result);
}
// If you try to set a value of a property that is not
// defined in the class, this method is called.
public override bool TrySetMember(SetMemberBinder binder, object value)
{
// Converting the property name to lowercase so that
// property names become case-insensitive.
dictionary[binder.Name.ToLower()] = value;
// You can always add a value to a dictionary, so this method always returns true.
return true;
}
}
And this is how you can use your DynamicDictionary:
dynamic person = new DynamicDictionary();
// Adding new dynamic properties. The TrySetMember method is called.
person.FirstName = "Ellen";
person.LastName = "Adams";
Reflection is one way to get at it. https://stackoverflow.com/a/1954663/83250 actually answers this perfectly. I would however restructure your data so the mechDef object is another dictionary but if you must keep it like your question asks, this will work:
void Main()
{
Dictionary<string, bool> limbRepair = new Dictionary<string, bool>
{
{ "Head", false },
{ "LeftArm", false },
{ "RightArm", false },
// Etc.
};
MechDefinition mechDef = new MechDefinition();
List<Limb> limbs = new List<Limb>();
foreach (KeyValuePair<string, bool> limbsToRepair in limbRepair.Where(x => !x.Value))
{
Limb limb = mechDef.GetPropValue<Limb>(limbsToRepair.Key);
limb.CurrentInternalStructure = 9001;
}
}
public class MechDefinition
{
public MechDefinition()
{
Head = new Limb
{
Id = Guid.NewGuid(),
DateAdded = DateTime.Parse("2018-01-01"),
Name = "Main Head",
CurrentInternalStructure = 8675309
};
}
public Guid Id { get; set; }
public string Name { get; set; }
public int CurrentInternalStructure { get; set; }
public Limb Head { get; set; } = new Limb();
public Limb LeftArm { get; set; } = new Limb();
public Limb RightArm { get; set; } = new Limb();
// etc...
}
public class Limb
{
public Guid Id { get; set; }
public string Name { get; set; }
public DateTime DateAdded { get; set; }
public int CurrentInternalStructure { get; set; }
public bool IsDisabled { get; set; }
}
public static class ReflectionHelpers
{
public static object GetPropValue(this object obj, string name)
{
foreach (string part in name.Split('.'))
{
if (obj == null) { return null; }
Type type = obj.GetType();
PropertyInfo info = type.GetProperty(part);
if (info == null) { return null; }
obj = info.GetValue(obj, null);
}
return obj;
}
public static T GetPropValue<T>(this object obj, string name)
{
object retval = GetPropValue(obj, name);
if (retval == null) { return default(T); }
// throws InvalidCastException if types are incompatible
return (T)retval;
}
}
Be aware that reflection is a very costly operation. If you are dealing with large sets of data, it will be very inefficient. Take a look at https://stackoverflow.com/a/7478557/83250 for a performance overview.
Also code-wise, I prefer to stay away from dynamic and reflection altogether. Reflection has its perks when you need to access a property attribute and dynamic is great if you don't have a strongly typed object. With that said, C# is a strongly typed language and should be treated as such whenever possible. By restructuring your mechDef as a Dictionary<string, Limb> object or something similar you will have a more efficient application.
If I understand correctly, You have something like this:
class LocationLoadoutDef
{
public LocationLoadoutDef()
{
Head = new Prop();
LeftArm = new Prop();
RightArm = new Prop();
CenterTorso = new Prop();
LeftTorso = new Prop();
RightTorso = new Prop();
LeftLeg = new Prop();
RightLeg = new Prop();
}
public Prop Head { get; set; }
public Prop LeftArm { get; set; }
public Prop RightArm { get; set; }
public Prop CenterTorso { get; set; }
public Prop LeftTorso { get; set; }
public Prop RightTorso { get; set; }
public Prop LeftLeg { get; set; }
public Prop RightLeg { get; set; }
...
}
class Prop
{
public float CurrentInternalStructure { get; set; }
...
}
So you can use reflection getting the type of the object and the property.
This is an example based on your pseudocode:
// your instance of LocationLoadoutDef
var mechDef = new LocationLoadoutDef();
//For reflection you need obtain the type
Type mechType = mechDef.GetType();
// loop your Dictionary
foreach (string limb in LimbRepair.Keys)
{
// If the property is false in the dictionary and the type has a property with that name
if (!LimbRepair[limb] && mechType.GetProperties().Any(p => p.Name == limb))
{
// Obtain the instance of the property
var property = mechType.GetProperty(limb).GetValue(mechDef) ;
// Get the property type
Type propertyType = property.GetType();
// If the property has a property CurrentInternalStructure
if (propertyType.GetProperties().Any(p => p.Name == "CurrentInternalStructure"))
{
// Obtain the current value for CurrentInternalStructure
var currentValue = propertyType.GetProperty("CurrentInternalStructure").GetValue(property);
// calculate the new value (I don't know what is rng)
var newValue = 1f ; //Math.Max(1f, (float)currentValue * (float)rng.NextDouble());
// set de value in the property
propertyType.GetProperty("CurrentInternalStructure").SetValue(property, newValue);
}
}
}
You can always create classic and working if .. else or switch.
Or create dictionary with function to update correct property
public class Repair
{
public bool Active { get; set; }
public Action<MechDef> Update { get; set; }
}
public class Settings
{
public readonly Dictionary<string, Repair> LimbRepair =
new Dictionary<string, bool> {
{
"Head",
new Repair { Active = false, mechDef => mechDef.Head.CurrentInternalStructure = yourFunctionForHead }
},
{
"LeftArm",
new Repair { Active = false, mechDef => mechDef.LeftArm.CurrentInternalStructure = yourFunctionForLeftArm }
},
// ... and so on
};
}
Then in the loop you will call correct update action, become much cleaner to use settings class with benefits of strong types and compiler help which prevent dynamic runtime errors
var updates = settings.LimbRepair.Where(pair => pair.Value.Active == false)
.Select(pair => pair.Value);
foreach (var repair in updates)
{
repair.Update();
}
Using C#...
Is there any way to specify property names for a projection function on a LINQ select method, from an array.
public class Album
{
public int Id { get; set; }
public string Name { get; set; }
public short Rate { get; set; }
public string Genre { get; set; }
public short Tracks { get; set; }
}
public class Class1
{
private void Some<T>()
{
// Example of source
var names = new[] { "Id", "Name", "Tracks" };
var query = myDataContext.
GetTable<T>.
AsQueryable().
Select( /* dynamic projection from names array */ );
// something like
// Select(x => new
// {
// x.Id,
// x.Name,
// x.Tracks
// }
GoAndDoSomethingWith(query);
}
}
Could this be done without System.Linq.Dynamic?
You could use reflection and dynamic types to generate an object with only the specified fields/properties.
Below is a simple way of doing this. You can do optimizations, like having a type cache for the reflection. But this should work for simple fields/properties.
public static object DynamicProjection(object input, IEnumerable<string> properties)
{
var type = input.GetType();
dynamic dObject = new ExpandoObject();
var dDict = dObject as IDictionary<string, object>;
foreach (var p in properties)
{
var field = type.GetField(p);
if (field != null)
dDict [p] = field.GetValue(input);
var prop = type.GetProperty(p);
if (prop != null && prop.GetIndexParameters().Length == 0)
dDict[p] = prop.GetValue(input, null);
}
return dObject;
}
Usage:
//...
var names = new[] { "Id", "Name", "Tracks" };
var projection = collection.Select(x => DynamicProjection(x, names));
//...
I have a class, which is created and populated from an xml string, I've simplified it for example purposes:
[XmlRoot("Person")]
public sealed class Person
{
[XmlElement("Name")]
public string Name { get; set; }
[XmlElement("Location")]
public string Location { get; set; }
[XmlElement("Emails", Type = typeof(PersonEmails)]
public PersonEmails Emails { get; set; }
}
public class PersonEmails
{
[XmlElement("Email", Type = typeof(PersonEmail))]
public PersonEmail[] Emails { get; set; }
}
public class PersonEmail
{
[XmlAttribute("Type")]
public string Type { get; set; }
[XmlText]
public string Value { get; set; }
}
To extract the information, I'm trying to load them into another class, which is simply:
public class TransferObject
{
public string Name { get; set; }
public ObjectField[] Fields { get; set; }
}
public class ObjectField
{
public string Name { get; set; }
public string Value { get; set; }
}
I'm only populating "Fields" from the other object, which would simply be (Name = "Location", Value = "London"), but for Emails, (Name = "Email"+Type, Value = jeff#here.com)
Currently I can populate all the other fields, but I'm stuck with Emails, and knowing how to dig deep enough to be able to use reflection (or not) to get the information I need. Currently I'm using:
Person person = Person.FromXmlString(xmlString);
List<ObjectField> fields = new List<ObjectField>();
foreach (PropertyInfo pinfo in person.getType().GetProperties()
{
fields.Add(new ObjectField { Name = pinfo.Name, Value = pinfo.getValue(person, null).ToString();
}
How can I expand on the above to add all my emails to the list?
You are trying to type cast a complex values type to string value so you lost the data. Instead use following code:
class Program
{
static void Main(string[] args)
{
Person person = new Person();
person.Name = "Person One";
person.Location = "India";
person.Emails = new PersonEmails();
person.Phones = new PersonPhones();
person.Emails.Emails = new PersonEmail[] { new PersonEmail() { Type = "Official", Value = "xyz#official.com" }, new PersonEmail() { Type = "Personal", Value = "xyz#personal.com" } };
person.Phones.Phones = new PersonPhone[] { new PersonPhone() { Type = "Official", Value = "789-456-1230" }, new PersonPhone() { Type = "Personal", Value = "123-456-7890" } };
List<ObjectField> fields = new List<ObjectField>();
fields = GetPropertyValues(person);
}
static List<ObjectField> GetPropertyValues(object obj)
{
List<ObjectField> propList = new List<ObjectField>();
foreach (PropertyInfo pinfo in obj.GetType().GetProperties())
{
var value = pinfo.GetValue(obj, null);
if (pinfo.PropertyType.IsArray)
{
var arr = value as object[];
for (var i = 0; i < arr.Length; i++)
{
if (arr[i].GetType().IsPrimitive)
{
propList.Add(new ObjectField() { Name = pinfo.Name + i.ToString(), Value = arr[i].ToString() });
}
else
{
var lst = GetPropertyValues(arr[i]);
if (lst != null && lst.Count > 0)
propList.AddRange(lst);
}
}
}
else
{
if (pinfo.PropertyType.IsPrimitive || value.GetType() == typeof(string))
{
propList.Add(new ObjectField() { Name = pinfo.Name, Value = value.ToString() });
}
else
{
var lst = GetPropertyValues(value);
if (lst != null && lst.Count > 0)
propList.AddRange(lst);
}
}
}
return propList;
}
}
Check this snippet out:
if(pinfo.PropertyType.IsArray)
{
// Grab the actual instance of the array.
// We'll have to use it in a few spots.
var array = pinfo.GetValue(personObject);
// Get the length of the array and build an indexArray.
int length = (int)pinfo.PropertyType.GetProperty("Length").GetValue(array);
// Get the "GetValue" method so we can extact the array values
var getValue = findGetValue(pinfo.PropertyType);
// Cycle through each index and use our "getValue" to fetch the value from the array.
for(int i=0; i<length; i++)
fields.Add(new ObjectField { Name = pinfo.Name, Value = getValue.Invoke(array, new object[]{i}).ToString();
}
// Looks for the "GetValue(int index)" MethodInfo.
private static System.Reflection.MethodInfo findGetValue(Type t)
{
return (from mi in t.GetMethods()
where mi.Name == "GetValue"
let parms = mi.GetParameters()
where parms.Length == 1
from p in parms
where p.ParameterType == typeof(int)
select mi).First();
}
You can definately do it with Reflection... You can take advantage of the fact that a Type can tell you if it's an array or not (IsArray)... and then take advantage of the fact that an Array has a method GetValue(int index) that will give you a value back.
Per your comment
Because Emails is a property within a different class, recursion should be used. However the trick is knowing when to go to the next level. Really that is up to you, but
if it were me, I would use some sort of Attribute:
static void fetchProperties(Object instance, List<ObjectField> fields)
{
foreach(var pinfo in instance.GetType().GetProperties())
{
if(pinfo.PropertyType.IsArray)
{
... // Code described above
}
else if(pinfo.PropertyType.GetCustomAttributes(typeof(SomeAttribute), false).Any())
// Go the next level
fetchProperties(pinfo.GetValue(instance), fields);
else
{
... // Do normal code
}
}
}
I have this :
public class Company
{
public int Id { get; set; }
public string Name { get; set; }
}
public class City
{
public int Id { get; set; }
public string Name { get; set; }
public int ZipCode { get; set; }
}
public class Person
{
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public int? Age { get; set; }
public City City { get; set; }
public Company Company { get; set; }
}
I'd like a some case generate the predicate like this :
var result = listPerson.Where(x => x.Age == 10).ToList<>();
Or this :
var result = listPerson.Where( x => x.Company.Name == 1234).ToList();
Or this :
var result = listPerson.Where( x => x.City.ZipCode == "MyZipCode").ToList();
Or this :
var result = listPerson.Where( x => x.Company.Name == "MyCompanyName").ToList();
Then I created a "PredicateBuilder", that's work (I get the type, if nullable or not and I build the predicate) when I do this :
BuildPredicate<Person>("Age", 10); I get this : x => x.Age == 10
But I don't how manage when there is an nested property like this :
BuildPredicate<Person>("City.ZipCode", "MyZipCode");
I'd like get this : x => x.City.ZipCode == "MyZipCode"
Or this :
BuildPredicate<Person>("City.Name", "MyName");
I'd like get this : x => x.City.Name == "MyName"
Or this :
BuildPredicate<Person>("Company.Name", "MyCompanyName");
I'd like get this : x => x.Company.Name == "MyCompanyName"
(not intending to duplicate Jon - OP contacted me to provide an answer)
The following seems to work fine:
static Expression<Func<T,bool>> BuildPredicate<T>(string member, object value) {
var p = Expression.Parameter(typeof(T));
Expression body = p;
foreach (var subMember in member.Split('.')) {
body = Expression.PropertyOrField(body, subMember);
}
return Expression.Lambda<Func<T, bool>>(Expression.Equal(
body, Expression.Constant(value, body.Type)), p);
}
The only functional difference between that and Jon's answer is that it handles null slightly better, by telling Expression.Constant what the expected type is. As a demonstration of usage:
static void Main() {
var pred = BuildPredicate<Person>("City.Name", "MyCity");
var people = new[] {
new Person { City = new City { Name = "Somewhere Else"} },
new Person { City = new City { Name = "MyCity"} },
};
var person = people.AsQueryable().Single(pred);
}
You just need to split your expression by dots, and then iterate over it, using Expression.Property multiple times. Something like this:
string[] properties = path.Split('.');
var parameter = Expression.Parameter(typeof(T), "x");
var lhs = parameter;
foreach (var property in properties)
{
lhs = Expression.Property(lhs, property);
}
// I've assumed that the target is a string, given the question. If that's
// not the case, look at Marc's answer.
var rhs = Expression.Constant(targetValue, typeof(string));
var predicate = Expression.Equals(lhs, rhs);
var lambda = Expression.Lambda<Func<T, bool>>(predicate, parameter);