Check via Reflection if Property is computed

Question

Please imagine having following two properties inside a class:

public string Category { get; set; }
public string DisplayCategory => "SomeCategory"

Now I just want to collect all PropertyInfo objects where the property itself is not computed

var properties = type.GetProperties();
var serializables = properties.Where(p => p.CanRead, true));

How do I find out via Reflection if a property is a computed one so I can ignore it?

The reason I want to do this, is because I use Expression Trees to automatically create queries that are processed via Entity Framework 6 . Entity Framework only creates columns for non-computed properties and therefore one can't query the computed ones.

See This Article

Answer 1

I use DelegateDecompiler and currently I have to manually add each computed property name to a custom configuration.

It would be nice if the ShouldDecompile method below could automatically determine which properties should be decompiled:

  public class CustomDelegateDecompilerConfiguration : Configuration {
    public static CustomDelegateDecompilerConfiguration Instance { get; } = new CustomDelegateDecompilerConfiguration();
    public static void Enable() => Configuration.Configure(Instance);

    public CustomDelegateDecompilerConfiguration() {
      RegisterDecompileableMember<Person, string>(x => x.Name);

      RegisterDecompileableMembers(typeof(string), nameof(string.IsNullOrWhiteSpace));

      RegisterDecompileableMembers(typeof(CustomComputedMethods), new[] {
        nameof(CustomComputedMethods.PersonName),
        nameof(CustomComputedMethods.MonthInteger),
        nameof(CustomComputedMethods.WholeMonthsBetween),
        nameof(CustomComputedMethods.WholeYearsBetween)
      });

    }

    public HashSet<MemberInfo> DecompileableMembers { get; } = new HashSet<MemberInfo>();

    public override bool ShouldDecompile(MemberInfo memberInfo) => memberInfo.GetCustomAttributes(typeof(DecompileAttribute), true).Length > 0
      || memberInfo.GetCustomAttributes(typeof(ComputedAttribute), true).Length > 0
      || memberInfo.GetCustomAttributes(typeof(CompilerGeneratedAttribute), true).Length > 0
      || DecompileableMembers.Contains(memberInfo)
      //***  Would be nice if auto detection was possible with non-existing methods below ***
      //|| memberInfo.AutoProperty      (One with a backing field automatically generated by the compiler)
      //|| memberInfo.HasExpressionBody (One implemented using the => (lambda) syntax)
      //|| memberInfo.HasFunctionBody   (One implemented using the normal {...} syntax)
      ;

    public override void RegisterDecompileableMember(MemberInfo prop) => DecompileableMembers.Add(prop);

    public void RegisterDecompileableMember<T, TResult>(Expression<Func<T, TResult>> expression) where T : class => RegisterDecompileableMember(expression.Body.GetMemberInfo());

    public void RegisterDecompileableMembers(Type type, params string[] memberNames) {
      foreach(var tmi in type.GetMembers().Where(mi => memberNames.Contains(mi.Name))) {
        DecompileableMembers.Add(tmi);
      }
    }

    public void RegisterDecompileableMembers<T>(params string[] memberNames) where T : class => RegisterDecompileableMembers(typeof(T), memberNames);

  }

An example class:

  public class Person {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string AlternativeFirstName { get; set; }

    public string Name => string.Concat(AlternativeFirstName == string.Empty ? FirstName : AlternativeFirstName, " ", LastName);
  }

Some example extension Methods:

  public static class CustomComputedMethods {
    public static string PersonName(string firstName, string lastName, string knownAs) => (knownAs ?? firstName).Trim() + " " + lastName.Trim();
    public static long MonthInteger(this DateTime d) => checked(d.Year * 12 + d.Month);
    public static int WholeMonthsBetween(this DateTime d, DateTime maxDate) => (int)(maxDate.MonthInteger() - d.MonthInteger() - (d.Day > maxDate.Day ? 1 : 0));
    public static int WholeYearsBetween(this DateTime d, DateTime maxDate) => d.WholeMonthsBetween(maxDate) / 12;
  }

Answer 2

The below worked for me.

I was trying to see if I could find a difference between a computed property, and a property with no Set method:

public int Computed => 2;
public int NoSet { get; }

PropertyInfo.SetMethod returns null for both.

Looking at the generated IL of the get method, There is a CompilerGeneratedAttribute on the NoSet property, but not on the Computed property:

.method public hidebysig specialname instance int32
  get_Computed() cil managed
{
  .maxstack 8

  // [215 32 - 215 33]
  IL_0000: ldc.i4.2
  IL_0001: ret
}

.method public hidebysig specialname instance int32
  get_NoSet() cil managed
{
  .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor()
    = (01 00 00 00 )
  .maxstack 8

  // [217 28 - 217 32]
  IL_0000: ldarg.0      // this
  IL_0001: ldfld        int32 TestNamespace.TestClass::'<NoSet>k__BackingField'
  IL_0006: ret

}

So the check becomes:

if (property.SetMethod == null && property.GetMethod.GetCustomAttribute(typeof(CompilerGeneratedAttribute)) == null)
    return "Computed Property";