Question

For example, let’s say i have a string property “Document.DocumentType.DocumentCode” with DocumentCode is a nullable decimal type.

How can I build an expression tree for this: x.Document.DocumentType.DocumentCode.GetValueOrDefault() == 4?

For my real case, I won’t know exactly what the linked string property will look like or the levels of properties.

Answer 1

Well, that tree is:

• An equality match, where the left hand side is complicated and the right hand side is a constant expression 4.
• The LHS is a method call GetValueOrDefault() on (an expression)
• The expression from the previous step is a property access expression DocumentCode on (an expression)
• The expression from the previous step is a property access expression DocumentType on (an expression)
• The expression from the previous step is a property access expression Document on a ParameterExpression

Start from the bottom, and build it up from there. In other words, if you know you’re only going to have properties, you’ll want to:

• Split the string by “.”
• Create a ParameterExpression

• Loop round the set of properties, adding another layer or property access each time, e.g.

  currentExpression = Expression.Property(currentExpression, propertyName);
  

• Add a method call at to the expression

• Build an equality comparison using that and whatever constant value you’re given