I have a expression to point on a property of my type. But it does not work for every property type. "Does not mean" means it result in different expression types. I thought it will ever result in a MemberExpression
but this is not the case.
For int
and Guid
it results in a UnaryExpression
and for string
in a MemberExpression
.
I am a little confused ;)
My class
public class Person { public string Name { get; set; } public int Age { get; set; } }
Test Code
Person p = new Person { Age = 16, Name = "John" }; Expression<Func<Person, object>> expression1 = x => x.Age; // expression1.Body = UnaryExpression; Expression<Func<Person, object>> expression2 = x => x.Name; // expression2.Body = MemberExpression;
How can i compare two expressions and check if they are mean the same type and same property ?
Thanks to user dasblinkenlight who brought me on the right track.
He provided the method
private static MemberExpression GetMemberExpression<T>( Expression<Func<T,object>> exp ) { var member = expr.Body as MemberExpression; var unary = expr.Body as UnaryExpression; return member ?? (unary != null ? unary.Operand as MemberExpression : null); }
I wrote the following extension method to compare the results of the GetMemberExpression
methods and check if GetMemberExpression().Member.Name
are the same.
private static bool IsSameMember<T>(this Expression<Func<T, object>> expr1, Expression<Func<T, object>> expr2) { var result1 = GetMemberExpression(expr1); var result2 = GetMemberExpression(expr2); if (result1 == null || result2 == null) return false; return result1.Member.Name == result2.Member.Name; }
The reason this happens is that Age
is a value type. In order to coerce an expression returning a value type into Func<Person,object>
the compiler needs to insert a Convert(expr, typeof(object))
, a UnaryExpression
.
For string
s and other reference types, however, there is no need to box, so a "straight" member expression is returned.
If you would like to get to the MemberExpression
inside the UnaryExpression
, you can get its operand:
private static MemberExpression GetMemberExpression<T>( Expression<Func<T,object>> exp ) { var member = exp.Body as MemberExpression; var unary = exp.Body as UnaryExpression; return member ?? (unary != null ? unary.Operand as MemberExpression : null); }
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