fix(core): Fixed issue with DisplayValue helper functions (#3575)

fix(core): Fixed issue with DispalyValue helper functions

Core/Core/Models/Extensions/BaseExtensions.cs

@@ -201,6 +201,7 @@ public static bool IsDisplayableObject(this Base speckleObject)
     {
       T b => new List<T> { b },
       IReadOnlyList<T> list => list,
+      IEnumerable enumerable => enumerable.OfType<T>().ToList(),
       _ => null
     };
   }

Core/Tests/Speckle.Core.Tests.Unit/Models/Extensions/DisplayValueTests.cs

@@ -0,0 +1,43 @@
+using NUnit.Framework;
+using Speckle.Core.Models;
+using Speckle.Core.Models.Extensions;
+
+namespace Speckle.Core.Tests.Unit.Models.Extensions;
+
+[TestOf(typeof(BaseExtensions))]
+public class DisplayValueTests
+{
+  private const string PAYLOAD = "This is my payload";
+  private static readonly Base s_displayValue = new() { applicationId = PAYLOAD };
+
+  [TestCaseSource(nameof(TestCases))]
+  public void TestTryGetDisplayValue_WithValue(Base testCase)
+  {
+    var res = testCase.TryGetDisplayValue();
+
+    Assert.That(res, Has.Count.EqualTo(1));
+    Assert.That(res, Has.One.Items.TypeOf<Base>().With.Property(nameof(Base.applicationId)).EqualTo(PAYLOAD));
+  }
+
+  public static IEnumerable<Base> TestCases()
+  {
+    var listOfBase = new List<object> { s_displayValue }; //This is what our deserializer will output
+    var listOfMesh = new List<Base> { s_displayValue };
+    yield return new Base { ["@displayValue"] = s_displayValue };
+    yield return new Base { ["@displayValue"] = s_displayValue };
+    yield return new Base { ["displayValue"] = listOfBase };
+    yield return new Base { ["displayValue"] = listOfBase };
+    yield return new TypedDisplayValue { displayValue = s_displayValue };
+    yield return new TypedDisplayValueList { displayValue = listOfMesh };
+  }
+
+  private class TypedDisplayValue : Base
+  {
+    public Base displayValue { get; set; }
+  }
+
+  private class TypedDisplayValueList : Base
+  {
+    public List<Base> displayValue { get; set; }
+  }
+}