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Combine Expression (Expression<Func<TIn,TOut>> with Expression<Func<TOut, bool>>)

I have two expressions:
Expression<Func<T1, T2>>
Expression<Func<T2, bool>>
And I want to combine these and get a new expression of type Expression<Func<T1, bool>> so that it can be used in Entity Framework LINQ.
I combine them with help of Expression.Invoke(), but it doesn't work.
//Extensinon method
public static Expression<Func<T1, bool>> Compose<T1, T2>(this Expression<Func<T1, T2>> convertExpr, Expression<Func<T2, bool>> predicate)
=> Expression.Lambda<Func<T1, bool>>(Expression.Invoke(predicate, convertExpr.Body), convertExpr.Parameters.First());
...
Expression<Func<One, Two>> convert;
Expression<Func<Two, bool>> predicate;
Expression<Func<One, bool>> filter=convert.Compose(predicate);
// Works fine
List<One> lst;
lst.AsQueryable().Where(filter);
DbSet<One> src;
// In next line throws runtime NotSupportedException: The LINQ expression node type 'Invoke' is not supported in LINQ to Entities
src.Where(filter);
UPDATE
For example:
public class Book
{
public int Id {get; protected set;}
public string Name {get; protected set;}
public virtual Genre {get; protected set;}
}
public class Genre
{
public int Id {get; protected set;}
public string Name {get; protected set;}
}
...
Expression<Func<Book, Genre>> convert = b=>b.Genre;
Expression<Func<Genre, bool>> predicate = g=>g.Name=="fantasy";
// Need: b=>b.Genre=="fantasy"
Expression<Func<Genre, bool>> filter=convert.Compose(predicate);
// Works fine
List<Book> lst;
lst.AsQueryable().Where(filter);
DbSet<Book> books;
// In next line throws runtime NotSupportedException: The LINQ expression node type 'Invoke' is not supported in LINQ to Entities
books.Where(filter);

Using the common ExpressionVisitor for replacing one Expression with another, my standard Compose function (in the more common mathematical order, I think), substitutes the Body of one LambdaExpression for the parameter in another:
public static class ExpressionExt {
// Compose: (y => f(y)).Compose(x => g(x)) -> x => f(g(x))
/// <summary>
/// Composes two LambdaExpression into a new LambdaExpression
/// </summary>
/// <param name="Tpg">Type of parameter to gFn, and type of parameter to result lambda.</param>
/// <param name="Tpf">Type of result of gFn and type of parameter to fFn.</param>
/// <param name="TRes">Type of result of fFn and type of result of result lambda.</param>
/// <param name="fFn">The outer LambdaExpression.</param>
/// <param name="gFn">The inner LambdaExpression.</param>
/// <returns>LambdaExpression representing outer composed with inner</returns>
public static Expression<Func<Tpg, TRes>> Compose<Tpg, Tpf, TRes>(this Expression<Func<Tpf, TRes>> fFn, Expression<Func<Tpg, Tpf>> gFn) =>
Expression.Lambda<Func<Tpg, TRes>>(fFn.Body.Replace(fFn.Parameters[0], gFn.Body), gFn.Parameters[0]);
/// <summary>
/// Replaces an Expression (reference Equals) with another Expression
/// </summary>
/// <param name="orig">The original Expression.</param>
/// <param name="from">The from Expression.</param>
/// <param name="to">The to Expression.</param>
/// <returns>Expression with all occurrences of from replaced with to</returns>
public static Expression Replace(this Expression orig, Expression from, Expression to) => new ReplaceVisitor(from, to).Visit(orig);
}
/// <summary>
/// ExpressionVisitor to replace an Expression (that is Equals) with another Expression.
/// </summary>
public class ReplaceVisitor : ExpressionVisitor {
readonly Expression from;
readonly Expression to;
public ReplaceVisitor(Expression from, Expression to) {
this.from = from;
this.to = to;
}
public override Expression Visit(Expression node) => node == from ? to : base.Visit(node);
}
With this available, your example is straightforward:
Expression<Func<One, Two>> convert = p1 => new Two(p1);
Expression<Func<Two, bool>> predicate = p2 => p2 == new Two();
Expression<Func<One, bool>> filter = predicate.Compose(convert);
However, especially with EF expressions, it may be preferable to use my replacement for Invoke, which handles null propogation, in cases where your arguments may be null. It also uses the same Replace ExpressionVisitor as above:
public static class ExpressionExt2 {
public static Expression PropagateNull(this Expression orig) => new NullVisitor().Visit(orig);
// Apply: (x => f).Apply(args)
/// <summary>
/// Substitutes an array of Expression args for the parameters of a lambda, returning a new Expression
/// </summary>
/// <param name="e">The original LambdaExpression to "call".</param>
/// <param name="args">The Expression[] of values to substitute for the parameters of e.</param>
/// <returns>Expression representing e.Body with args substituted in</returns>
public static Expression Apply(this LambdaExpression e, params Expression[] args) {
var b = e.Body;
foreach (var pa in e.Parameters.Zip(args, (p, a) => (p, a)))
b = b.Replace(pa.p, pa.a);
return b.PropagateNull();
}
}
/// <summary>
/// ExpressionVisitor to replace a null.member Expression with a null
/// </summary>
public class NullVisitor : System.Linq.Expressions.ExpressionVisitor {
public override Expression Visit(Expression node) {
if (node is MemberExpression nme && nme.Expression is ConstantExpression nce && nce.Value == null)
return Expression.Constant(null, nce.Type.GetMember(nme.Member.Name).Single().GetMemberType());
else
return base.Visit(node);
}
}
public static class MeberInfoExt {
public static Type GetMemberType(this MemberInfo member) {
switch (member) {
case FieldInfo mfi:
return mfi.FieldType;
case PropertyInfo mpi:
return mpi.PropertyType;
case EventInfo mei:
return mei.EventHandlerType;
default:
throw new ArgumentException("MemberInfo must be if type FieldInfo, PropertyInfo or EventInfo", nameof(member));
}
}
}
Given Apply, your Compose is simply:
public static Expression<Func<T1, bool>> Compose<T1, T2>(this Expression<Func<T1, T2>> convertExpr, Expression<Func<T2, bool>> predicate)
=> Expression.Lambda<Func<T1, bool>>(predicate.Apply(convertExpr.Body), convertExpr.Parameters.First());

Write extension method to retrieve property description using c#

I have following class
public class Device
{
[XmlElement("MobileDeviceType")]
public string DeviceType { get; set; }
}
I need extension method called "GetXElementName()" and I need to use the method like below.
string propertyDescription = (new Device()).DeviceType.GetXElementName(); // this shoud return "MobileDeviceType"
As a example
public static class ExtensionMethods
{
public static string GetXElementName<T>(this T source)
{
PropertyInfo prop = source.GetType().GetProperty(source.ToString());
string desc = prop.Name;
object[] attrs = prop.GetCustomAttributes(true);
object attr = attrs[0];
XmlElementAttribute descAttr = attr as XmlElementAttribute;
if (descAttr != null)
{
desc = descAttr.ElementName;
}
return desc;
}
}
Can I know how should I modify the method body to use the "GetXElementName()" method to use like I explained above.

You need to use Expressions to achieve that, because you need to know the member, not the value.
public static class Extensions
{
public static string GetXmlElementName<T, TProperty>(this T obj, Expression<Func<T, TProperty>> expression)
{
var memberExpression = expression.Body as MemberExpression;
if (memberExpression == null)
return string.Empty;
var xmlElementAttribute = memberExpression.Member.GetCustomAttribute<XmlElementAttribute>();
if (xmlElementAttribute == null)
return string.Empty;
return xmlElementAttribute.ElementName;
}
}
Usage:
public class MyClass
{
[XmlElement(ElementName = "Test")]
public string MyProperty { get; set; }
}
new MyClass().GetXmlElementName(x => x.MyProperty) // output "Test"
EDIT: another version, without an object instance (see Nyerguds comment)
I guess the most elegant way is make a generic class, with a generic method, so you can call it by specify only the T type parameter (TProperty is taken implicitly).
public class GetXmlElementName<T>
{
public static string From<TProperty>(Expression<Func<T, TProperty>> expression)
{
var memberExpression = expression.Body as MemberExpression;
if (memberExpression == null)
return string.Empty;
var xmlElementAttribute = memberExpression.Member.GetCustomAttribute<XmlElementAttribute>();
if (xmlElementAttribute == null)
return string.Empty;
return xmlElementAttribute.ElementName;
}
}
Usage:
GetXmlElementName<MyClass>.From(x => x.MyProperty) // output "Test"

When using a repository is it possible for a type to return a Func that the repository uses to test for existing entities?

For example given a Factory with a method
public static T Save<T>(T item) where T : Base, new()
{
/* item.Id == Guid.Empty therefore item is new */
if (item.Id == Guid.Empty && repository.GetAll<T>(t => t.Name == item.Name))
{
throw new Exception("Name is not unique");
}
}
how do I create a property of Base (say MustNotAlreadyExist) so that I can change the method above to
public static T Save<T>(T item) where T : Base, new()
{
/* item.Id == Guid.Empty therefore item is new */
if (item.Id == Guid.Empty && repository.GetAll<T>(t.MustNotAlreadyExist))
{
throw new Exception("Name is not unique");
}
}
public class Base
{
...
public virtual Expression<Func<T, bool>> MustNotAlreadyExist()
{
return (b => b.Name == name); /* <- this clearly doesn't work */
}
}
and then how can I override MustNotAlreadyExist in Account : Base
public class Account : Base
{
...
public override Expression<Func<T, bool>> MustNotAlreadyExist()
{
return (b => b.Name == name && b.AccountCode == accountCode); /* <- this doesn't work */
}
...
}

Try this:
public class Account : Base
{
...
public override Expression<Func<T, bool>> MustNotAlreadyExist()
{
return (b => b.Name == name && b.AccountCode == accountCode).Any();
}
...
}
The Any() method will return true if any record matches the predicate. It could be argued that it is outside the responsibility of the repository to check for presence of a record before saving.
UPDATE:
There is a great article on CodeProject that describes a generic Repository for Entity Framework:
http://www.codeproject.com/KB/database/ImplRepositoryPatternEF.aspx
This could be applied to a non-Entity Framework data context. Here is an excerpt that provides a very flexible method for checking for an existing value by accepting the name of a field, a value, and a key value. You can apply this to any Entity type and use it to check for the presence of an entity before attempting a save.
/// <summary>
/// Check if value of specific field is already exist
/// </summary>
/// <typeparam name="E"></typeparam>
/// <param name="fieldName">name of the Field</param>
/// <param name="fieldValue">Field value</param>
/// <param name="key">Primary key value</param>
/// <returns>True or False</returns>
public bool TrySameValueExist(string fieldName, object fieldValue, string key)
{
// First we define the parameter that we are going to use the clause.
var xParam = Expression.Parameter(typeof(E), typeof(E).Name);
MemberExpression leftExprFieldCheck =
MemberExpression.Property(xParam, fieldName);
Expression rightExprFieldCheck = Expression.Constant(fieldValue);
BinaryExpression binaryExprFieldCheck =
MemberExpression.Equal(leftExprFieldCheck, rightExprFieldCheck);
MemberExpression leftExprKeyCheck =
MemberExpression.Property(xParam, this._KeyProperty);
Expression rightExprKeyCheck = Expression.Constant(key);
BinaryExpression binaryExprKeyCheck =
MemberExpression.NotEqual(leftExprKeyCheck, rightExprKeyCheck);
BinaryExpression finalBinaryExpr =
Expression.And(binaryExprFieldCheck, binaryExprKeyCheck);
//Create Lambda Expression for the selection
Expression<Func<E, bool>> lambdaExpr =
Expression.Lambda<Func<E, bool>>(finalBinaryExpr,
new ParameterExpression[] { xParam });
//Searching ....
return ((IRepository<E, C>)this).TryEntity(new Specification<E>(lambdaExpr));
}
/// <summary>
/// Check if Entities exist with Condition
/// </summary>
/// <param name="selectExpression">Selection Condition</param>
/// <returns>True or False</returns>
public bool TryEntity(ISpecification<E> selectSpec)
{
return _ctx.CreateQuery<E>("[" + typeof(E).Name + "]").Any<E>
(selectSpec.EvalPredicate);
}

I am not shure if you problem is solvable since you need to access both the repository and the new item to be checked. The new item to be checked is not available in a seperate method.
However you can outsource the call to GetAll so that your code becomes something similar to
(not tested)
public static T Save<T>(T item) where T : Base, new()
{
if (item.Id == Guid.Empty && (Check(repository, item)))
{
throw new Exception("Name is not unique");
}
}
public class Base
{
...
public Func<Enumerable<T>, T, bool> Check { get; set;}
public Base()
{
Check = (col, newItem) => (null != col.FirstOrDefault<T>(
item => item.Name == newItem.Name));
}
}

OK, here is the answer, this is a combination of the code posted by Dave Swersky and a little but of common sense.
public interface IUniqueable<T>
{
Expression<Func<T, bool>> Unique { get; }
}
public class Base, IUniqueable<Base>
{
...
public Expression<Func<Base, bool>> Unique
{
get
{
var xParam = Expression.Parameter(typeof(Base), typeof(Base).Name);
MemberExpression leftExprFieldCheck = MemberExpression.Property(xParam, "Name");
Expression rightExprFieldCheck = Expression.Constant(this.Name);
BinaryExpression binaryExprFieldCheck = MemberExpression.Equal(leftExprFieldCheck, rightExprFieldCheck);
return Expression.Lambda<Func<Base, bool>>(binaryExprFieldCheck, new ParameterExpression[] { xParam });
}
}
...
}
public class Account : Base, IUniqueable<Account>
{
...
public new Expression<Func<Account, bool>> Unique
{
get
{
var xParam = Expression.Parameter(typeof(Account), typeof(Account).Name);
MemberExpression leftExprNameCheck = MemberExpression.Property(xParam, "Name");
Expression rightExprNameCheck = Expression.Constant(this.Name);
BinaryExpression binaryExprNameCheck = MemberExpression.Equal(leftExprNameCheck, rightExprNameCheck);
MemberExpression leftExprFieldCheck = MemberExpression.Property(xParam, "AccountCode");
Expression rightExprFieldCheck = Expression.Constant(this.AccountCode);
BinaryExpression binaryExprFieldCheck = MemberExpression.Equal(leftExprFieldCheck, rightExprFieldCheck);
BinaryExpression binaryExprAllCheck = Expression.OrElse(binaryExprNameCheck, binaryExprFieldCheck);
return Expression.Lambda<Func<Account, bool>>(binaryExprAllCheck, new ParameterExpression[] { xParam });
}
}
...
}
public static class Manager
{
...
public static T Save<T>(T item) where T : Base, new()
{
if (!item.IsValid)
{
throw new ValidationException("Unable to save item, item is not valid", item.GetRuleViolations());
}
if (item.Id == Guid.Empty && repository.GetAll<T>().Any(((IUniqueable<T>)item).Unique))
{
throw new Exception("Item is not unique");
}
return repository.Save<T>(item);
}
...
}
Essentially by implementing the IUniqueable interface for a specific type I can return a different Expression for each type. All good :-)

Custom attribute on property - Getting type and value of attributed property

I have the following custom attribute, which can be applied on properties:
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class IdentifierAttribute : Attribute
{
}
For example:
public class MyClass
{
[Identifier()]
public string Name { get; set; }
public int SomeNumber { get; set; }
public string SomeOtherProperty { get; set; }
}
There will also be other classes, to which the Identifier attribute could be added to properties of different type:
public class MyOtherClass
{
public string Name { get; set; }
[Identifier()]
public int SomeNumber { get; set; }
public string SomeOtherProperty { get; set; }
}
I then need to be able to get this information in my consuming class.
For example:
public class TestClass<T>
{
public void GetIDForPassedInObject(T obj)
{
var type = obj.GetType();
//type.GetCustomAttributes(true)???
}
}
What's the best way of going about this?
I need to get the type of the [Identifier()] field (int, string, etc...) and the actual value, obviously based on the type.

Something like the following,, this will use only the first property it comes accross that has the attribute, of course you could place it on more than one..
public object GetIDForPassedInObject(T obj)
{
var prop = typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance)
.FirstOrDefault(p => p.GetCustomAttributes(typeof(IdentifierAttribute), false).Count() ==1);
object ret = prop !=null ? prop.GetValue(obj, null) : null;
return ret;
}

public class TestClass<T>
{
public void GetIDForPassedInObject(T obj)
{
PropertyInfo[] properties =
obj.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance);
PropertyInfo IdProperty = (from PropertyInfo property in properties
where property.GetCustomAttributes(typeof(Identifier), true).Length > 0
select property).First();
if(null == IdProperty)
throw new ArgumentException("obj does not have Identifier.");
Object propValue = IdProperty.GetValue(entity, null)
}
}

A bit late but here is something I did for enums (could be any object also) and getting the description attribute value using an extension (this could be a generic for any attribute):
public enum TransactionTypeEnum
{
[Description("Text here!")]
DROP = 1,
[Description("More text here!")]
PICKUP = 2,
...
}
Getting the value:
var code = TransactionTypeEnum.DROP.ToCode();
Extension supporting all my enums:
public static string ToCode(this TransactionTypeEnum val)
{
return GetCode(val);
}
public static string ToCode(this DockStatusEnum val)
{
return GetCode(val);
}
public static string ToCode(this TrailerStatusEnum val)
{
return GetCode(val);
}
public static string ToCode(this DockTrailerStatusEnum val)
{
return GetCode(val);
}
public static string ToCode(this EncodingType val)
{
return GetCode(val);
}
private static string GetCode(object val)
{
var attributes = (DescriptionAttribute[])val.GetType().GetField(val.ToString()).GetCustomAttributes(typeof(DescriptionAttribute), false);
return attributes.Length > 0 ? attributes[0].Description : string.Empty;
}

Here is a more real-word example. We use an extension method and check if a property contains a FieldMetaDataAttribute (a custom attribute in my source code base)
with valid Major and MinorVersion. What is of general interest is the part where we use the parent class type and GetProperties and retrieve the ProperyInfo and then use GetCustomAttribute to retrieve a attribute FieldMetaDataAttribute in this special case. Use this code for inspiration how to do more generic way of retrieving a custom attribute. This can of course be polished to make a general method to retrieve a given attribute of any property of a class instance.
/// <summary>
/// Executes the action if not the field is deprecated
/// </summary>
/// <typeparam name="TProperty"></typeparam>
/// <typeparam name="TForm"></typeparam>
/// <param name="form"></param>
/// <param name="memberExpression"></param>
/// <param name="actionToPerform"></param>
/// <returns>True if the action was performed</returns>
public static bool ExecuteActionIfNotDeprecated<TForm, TProperty>(this TForm form, Expression<Func<TForm, TProperty>> memberExpression, Action actionToPerform)
{
var memberExpressionConverted = memberExpression.Body as MemberExpression;
if (memberExpressionConverted == null)
return false;
string memberName = memberExpressionConverted.Member.Name;
PropertyInfo matchingProperty = typeof(TForm).GetProperties(BindingFlags.Public | BindingFlags.Instance)
.FirstOrDefault(p => p.Name == memberName);
if (matchingProperty == null)
return false; //should not occur
var fieldMeta = matchingProperty.GetCustomAttribute(typeof(FieldMetadataAttribute), true) as FieldMetadataAttribute;
if (fieldMeta == null)
{
actionToPerform();
return true;
}
var formConverted = form as FormDataContract;
if (formConverted == null)
return false;
if (fieldMeta.DeprecatedFromMajorVersion > 0 && formConverted.MajorVersion > fieldMeta.DeprecatedFromMajorVersion)
{
//major version of formConverted is deprecated for this field - do not execute action
return false;
}
if (fieldMeta.DeprecatedFromMinorVersion > 0 && fieldMeta.DeprecatedFromMajorVersion > 0
&& formConverted.MinorVersion >= fieldMeta.DeprecatedFromMinorVersion
&& formConverted.MajorVersion >= fieldMeta.DeprecatedFromMajorVersion)
return false; //the field is expired - do not invoke action
actionToPerform();
return true;
}

Get name of property as a string

(See below solution I created using the answer I accepted)
I'm trying to improve the maintainability of some code involving reflection. The app has a .NET Remoting interface exposing (among other things) a method called Execute for accessing parts of the app not included in its published remote interface.
Here is how the app designates properties (a static one in this example) which are meant to be accessible via Execute:
RemoteMgr.ExposeProperty("SomeSecret", typeof(SomeClass), "SomeProperty");
So a remote user could call:
string response = remoteObject.Execute("SomeSecret");
and the app would use reflection to find SomeClass.SomeProperty and return its value as a string.
Unfortunately, if someone renames SomeProperty and forgets to change the 3rd parm of ExposeProperty(), it breaks this mechanism.
I need to the equivalent of:
SomeClass.SomeProperty.GetTheNameOfThisPropertyAsAString()
to use as the 3rd parm in ExposeProperty so refactoring tools would take care of renames.
Is there a way to do this?
Okay, here's what I ended up creating (based upon the answer I selected and the question he referenced):
// <summary>
// Get the name of a static or instance property from a property access lambda.
// </summary>
// <typeparam name="T">Type of the property</typeparam>
// <param name="propertyLambda">lambda expression of the form: '() => Class.Property' or '() => object.Property'</param>
// <returns>The name of the property</returns>
public string GetPropertyName<T>(Expression<Func<T>> propertyLambda)
{
var me = propertyLambda.Body as MemberExpression;
if (me == null)
{
throw new ArgumentException("You must pass a lambda of the form: '() => Class.Property' or '() => object.Property'");
}
return me.Member.Name;
}
Usage:
// Static Property
string name = GetPropertyName(() => SomeClass.SomeProperty);
// Instance Property
string name = GetPropertyName(() => someObject.SomeProperty);
Now with this cool capability, it's time to simplify the ExposeProperty method. Polishing doorknobs is dangerous work...

With C# 6.0, this is now a non-issue as you can do:
nameof(SomeProperty)
This expression is resolved at compile-time to "SomeProperty".
MSDN documentation of nameof.

Using GetMemberInfo from here: Retrieving Property name from lambda expression you can do something like this:
RemoteMgr.ExposeProperty(() => SomeClass.SomeProperty)
public class SomeClass
{
public static string SomeProperty
{
get { return "Foo"; }
}
}
public class RemoteMgr
{
public static void ExposeProperty<T>(Expression<Func<T>> property)
{
var expression = GetMemberInfo(property);
string path = string.Concat(expression.Member.DeclaringType.FullName,
".", expression.Member.Name);
// Do ExposeProperty work here...
}
}
public class Program
{
public static void Main()
{
RemoteMgr.ExposeProperty("SomeSecret", () => SomeClass.SomeProperty);
}
}

Okay, here's what I ended up creating (based upon the answer I selected and the question he referenced):
// <summary>
// Get the name of a static or instance property from a property access lambda.
// </summary>
// <typeparam name="T">Type of the property</typeparam>
// <param name="propertyLambda">lambda expression of the form: '() => Class.Property' or '() => object.Property'</param>
// <returns>The name of the property</returns>
public string GetPropertyName<T>(Expression<Func<T>> propertyLambda)
{
var me = propertyLambda.Body as MemberExpression;
if (me == null)
{
throw new ArgumentException("You must pass a lambda of the form: '() => Class.Property' or '() => object.Property'");
}
return me.Member.Name;
}
Usage:
// Static Property
string name = GetPropertyName(() => SomeClass.SomeProperty);
// Instance Property
string name = GetPropertyName(() => someObject.SomeProperty);

There's a well-known hack to extract it from lambda expression (this is from the PropertyObserver class, by Josh Smith, in his MVVM foundation):
private static string GetPropertyName<TPropertySource>
(Expression<Func<TPropertySource, object>> expression)
{
var lambda = expression as LambdaExpression;
MemberExpression memberExpression;
if (lambda.Body is UnaryExpression)
{
var unaryExpression = lambda.Body as UnaryExpression;
memberExpression = unaryExpression.Operand as MemberExpression;
}
else
{
memberExpression = lambda.Body as MemberExpression;
}
Debug.Assert(memberExpression != null,
"Please provide a lambda expression like 'n => n.PropertyName'");
if (memberExpression != null)
{
var propertyInfo = memberExpression.Member as PropertyInfo;
return propertyInfo.Name;
}
return null;
}
Sorry, this was missing some context. This was part of a larger class where TPropertySource is the class containing the property. You could make the function generic in TPropertySource to extract it from the class. I recommend taking a look at the full code from the MVVM Foundation.

The PropertyInfo class should help you achieve this, if I understand correctly.
Type.GetProperties() method
PropertyInfo[] propInfos = typeof(ReflectedType).GetProperties();
propInfos.ToList().ForEach(p =>
Console.WriteLine(string.Format("Property name: {0}", p.Name));
Is this what you need?

You can use Reflection to obtain the actual names of the properties.
http://www.csharp-examples.net/reflection-property-names/
If you need a way to assign a "String Name" to a property, why don't you write an attribute that you can reflect over to get the string name?
[StringName("MyStringName")]
private string MyProperty
{
get { ... }
}

I modified your solution to chain over multiple properties:
public static string GetPropertyName<T>(Expression<Func<T>> propertyLambda)
{
MemberExpression me = propertyLambda.Body as MemberExpression;
if (me == null)
{
throw new ArgumentException("You must pass a lambda of the form: '() => Class.Property' or '() => object.Property'");
}
string result = string.Empty;
do
{
result = me.Member.Name + "." + result;
me = me.Expression as MemberExpression;
} while (me != null);
result = result.Remove(result.Length - 1); // remove the trailing "."
return result;
}
Usage:
string name = GetPropertyName(() => someObject.SomeProperty.SomeOtherProperty);
// returns "SomeProperty.SomeOtherProperty"

Old question, but another answer to this question is to create a static function in a helper class that uses the CallerMemberNameAttribute.
public static string GetPropertyName([CallerMemberName] String propertyName = null) {
return propertyName;
}
And then use it like:
public string MyProperty {
get { Console.WriteLine("{0} was called", GetPropertyName()); return _myProperty; }
}

Based on the answer which is already in the question and on this article: https://handcraftsman.wordpress.com/2008/11/11/how-to-get-c-property-names-without-magic-strings/ I am presenting my solution to this problem:
public static class PropertyNameHelper
{
/// <summary>
/// A static method to get the Propertyname String of a Property
/// It eliminates the need for "Magic Strings" and assures type safety when renaming properties.
/// See: http://stackoverflow.com/questions/2820660/get-name-of-property-as-a-string
/// </summary>
/// <example>
/// // Static Property
/// string name = PropertyNameHelper.GetPropertyName(() => SomeClass.SomeProperty);
/// // Instance Property
/// string name = PropertyNameHelper.GetPropertyName(() => someObject.SomeProperty);
/// </example>
/// <typeparam name="T"></typeparam>
/// <param name="propertyLambda"></param>
/// <returns></returns>
public static string GetPropertyName<T>(Expression<Func<T>> propertyLambda)
{
var me = propertyLambda.Body as MemberExpression;
if (me == null)
{
throw new ArgumentException("You must pass a lambda of the form: '() => Class.Property' or '() => object.Property'");
}
return me.Member.Name;
}
/// <summary>
/// Another way to get Instance Property names as strings.
/// With this method you don't need to create a instance first.
/// See the example.
/// See: https://handcraftsman.wordpress.com/2008/11/11/how-to-get-c-property-names-without-magic-strings/
/// </summary>
/// <example>
/// string name = PropertyNameHelper((Firma f) => f.Firmenumsatz_Waehrung);
/// </example>
/// <typeparam name="T"></typeparam>
/// <typeparam name="TReturn"></typeparam>
/// <param name="expression"></param>
/// <returns></returns>
public static string GetPropertyName<T, TReturn>(Expression<Func<T, TReturn>> expression)
{
MemberExpression body = (MemberExpression)expression.Body;
return body.Member.Name;
}
}
And a Test which also shows the usage for instance and static properties:
[TestClass]
public class PropertyNameHelperTest
{
private class TestClass
{
public static string StaticString { get; set; }
public string InstanceString { get; set; }
}
[TestMethod]
public void TestGetPropertyName()
{
Assert.AreEqual("StaticString", PropertyNameHelper.GetPropertyName(() => TestClass.StaticString));
Assert.AreEqual("InstanceString", PropertyNameHelper.GetPropertyName((TestClass t) => t.InstanceString));
}
}

You can use the StackTrace class to get the name of the current function, (or if you put the code in a function, then step down a level and get the calling function).
See http://msdn.microsoft.com/en-us/library/system.diagnostics.stacktrace(VS.71).aspx

I've been using this answer to great effect: Get the property, as a string, from an Expression<Func<TModel,TProperty>>
I realize I already answered this question a while back. The only advantage my other answer has is that it works for static properties. I find the syntax in this answer much more useful because you don't have to create a variable of the type you want to reflect.

I had some difficulty using the solutions already suggested for my specific use case, but figured it out eventually. I don't think my specific case is worthy of a new question, so I am posting my solution here for reference. (This is very closely related to the question and provides a solution for anyone else with a similar case to mine).
The code I ended up with looks like this:
public class HideableControl<T>: Control where T: class
{
private string _propertyName;
private PropertyInfo _propertyInfo;
public string PropertyName
{
get { return _propertyName; }
set
{
_propertyName = value;
_propertyInfo = typeof(T).GetProperty(value);
}
}
protected override bool GetIsVisible(IRenderContext context)
{
if (_propertyInfo == null)
return false;
var model = context.Get<T>();
if (model == null)
return false;
return (bool)_propertyInfo.GetValue(model, null);
}
protected void SetIsVisibleProperty(Expression<Func<T, bool>> propertyLambda)
{
var expression = propertyLambda.Body as MemberExpression;
if (expression == null)
throw new ArgumentException("You must pass a lambda of the form: 'vm => vm.Property'");
PropertyName = expression.Member.Name;
}
}
public interface ICompanyViewModel
{
string CompanyName { get; }
bool IsVisible { get; }
}
public class CompanyControl: HideableControl<ICompanyViewModel>
{
public CompanyControl()
{
SetIsVisibleProperty(vm => vm.IsVisible);
}
}
The important part for me is that in the CompanyControl class the compiler will only allow me to choose a boolean property of ICompanyViewModel which makes it easier for other developers to get it right.
The main difference between my solution and the accepted answer is that my class is generic and I only want to match properties from the generic type that are boolean.

it's how I implemented it , the reason behind is if the class that you want to get the name from it's member is not static then you need to create an instanse of that and then get the member's name. so generic here comes to help
public static string GetName<TClass>(Expression<Func<TClass, object>> exp)
{
MemberExpression body = exp.Body as MemberExpression;
if (body == null)
{
UnaryExpression ubody = (UnaryExpression)exp.Body;
body = ubody.Operand as MemberExpression;
}
return body.Member.Name;
}
the usage is like this
var label = ClassExtension.GetName<SomeClass>(x => x.Label); //x is refering to 'SomeClass'