[c#] handle abstract modifiers

joern-cli/frontends/csharpsrc2cpg/src/main/scala/io/joern/csharpsrc2cpg/astcreation/AstForDeclarationsCreator.scala

@@ -535,6 +535,7 @@ trait AstForDeclarationsCreator(implicit withSchemaValidation: ValidationMode) {
         case "readonly" => newModifierNode(ModifierTypes.READONLY)
         case "virtual"  => newModifierNode(ModifierTypes.VIRTUAL)
         case "const"    => newModifierNode(CSharpModifiers.CONST)
+        case "abstract" => newModifierNode(ModifierTypes.ABSTRACT)
         case x =>
           logger.warn(s"Unhandled modifier name '$x'")
           null

joern-cli/frontends/csharpsrc2cpg/src/test/scala/io/joern/csharpsrc2cpg/querying/ast/ClassDeclarationTests.scala

@@ -0,0 +1,22 @@
+package io.joern.csharpsrc2cpg.querying.ast
+
+import io.joern.csharpsrc2cpg.testfixtures.CSharpCode2CpgFixture
+import io.shiftleft.codepropertygraph.generated.ModifierTypes
+import io.shiftleft.semanticcpg.language.*
+
+class ClassDeclarationTests extends CSharpCode2CpgFixture {
+
+  "empty abstract class" should {
+    val cpg = code("""
+        |abstract class C {}
+        |""".stripMargin)
+
+    "have correct modifiers" in {
+      cpg.typeDecl.nameExact("C").modifier.modifierType.sorted.l shouldBe List(
+        ModifierTypes.ABSTRACT,
+        ModifierTypes.INTERNAL
+      )
+    }
+  }
+
+}

joern-cli/frontends/csharpsrc2cpg/src/test/scala/io/joern/csharpsrc2cpg/querying/ast/MethodTests.scala

@@ -77,4 +77,19 @@ class MethodTests extends CSharpCode2CpgFixture {
     }
   }
 
+  "empty public abstract method" should {
+    val cpg = code("""
+        |abstract class C
+        |{
+        | public abstract void DoStuff();
+        |}
+        |""".stripMargin)
+
+    "have correct modifiers" in {
+      cpg.method.nameExact("DoStuff").modifier.modifierType.sorted.l shouldBe List(
+        ModifierTypes.ABSTRACT,
+        ModifierTypes.PUBLIC
+      )
+    }
+  }
 }