foo1.cs

    [Pure, ContractAnnotation("null => null")]
    public static IArrowExpressionClause GetArrowExpressionClause([CanBeNull] this IExpressionBodyOwnerDeclaration declaration)
    {
      var functionDeclaration = declaration as ICSharpFunctionDeclaration;
      if (functionDeclaration != null) return functionDeclaration.ArrowExpression;

      var propertyDeclaration = declaration as IPropertyDeclaration;
      if (propertyDeclaration != null) return propertyDeclaration.ArrowExpression;

      var indexerDeclaration = declaration as IIndexerDeclaration;
      if (indexerDeclaration != null) return indexerDeclaration.ArrowExpression;

      var localFunctionDeclaration = declaration as ILocalFunctionDeclaration;
      if (localFunctionDeclaration != null) return localFunctionDeclaration.ArrowExpression;

      return null;
    }

foo2.cs

    [Pure, ContractAnnotation("null => null")]
    public static IArrowExpressionClause GetArrowExpressionClause([CanBeNull] this IExpressionBodyOwnerDeclaration declaration)
    {
      switch (declaration)
      {
        case ICSharpFunctionDeclaration functionDeclaration:
          return functionDeclaration.ArrowExpression;
        case IPropertyDeclaration propertyDeclaration:
          return propertyDeclaration.ArrowExpression;
        case IIndexerDeclaration indexerDeclaration:
          return indexerDeclaration.ArrowExpression;
        case ILocalFunctionDeclaration localFunctionDeclaration:
          return localFunctionDeclaration.ArrowExpression;
        default:
          return null;
      }
    }

foo3.cs

    [Pure, ContractAnnotation("null => null")]
    public static IArrowExpressionClause GetArrowExpressionClause([CanBeNull] this IExpressionBodyOwnerDeclaration declaration)
    {
      switch (declaration)
      {
        case ICSharpFunctionDeclaration functionDeclaration:     return functionDeclaration.ArrowExpression;
        case IPropertyDeclaration propertyDeclaration:           return propertyDeclaration.ArrowExpression;
        case IIndexerDeclaration indexerDeclaration:             return indexerDeclaration.ArrowExpression;
        case ILocalFunctionDeclaration localFunctionDeclaration: return localFunctionDeclaration.ArrowExpression;
        default: return null;
      }
    }

foo4.cs

    [Pure, ContractAnnotation("null => null")]
    public static IArrowExpressionClause GetArrowExpressionClause([CanBeNull] this IExpressionBodyOwnerDeclaration declaration)
    {
      return switch (declaration)
      {
        case ICSharpFunctionDeclaration functionDeclaration => functionDeclaration.ArrowExpression;
        case IPropertyDeclaration propertyDeclaration => propertyDeclaration.ArrowExpression;
        case IIndexerDeclaration indexerDeclaration => indexerDeclaration.ArrowExpression;
        case ILocalFunctionDeclaration localFunctionDeclaration => localFunctionDeclaration.ArrowExpression;
        default => null;
      }
    }