Project project = CreateTestProjectWithCustomField();

// Set arithmetic formula for extended attribute
ExtendedAttributeDefinition attr = project.ExtendedAttributes[0];
attr.Alias = "Arithmetic Expression";
attr.Formula = "(1+3*(2+ -5)+8/2)^3";

// Display extended attribute value
Task task = project.RootTask.Children.GetById(1);
Console.WriteLine(task.ExtendedAttributes[0].NumericValue);