C#: Problem trying to resolve a class when two namespaces are similar

You can use global:: to globally qualify a namespace: global::Foo.Class1 should work,.

You could also alias global::Foo to make things easier. At the top of your source file, below your using statements, add:

using AliasClass1=global::Foo.Class1;

Now you can use:

AliasClass1 c = new AliasClass1();
// and so on.

Of course, you can use a better name than AliasClass :-)

In addition to LBushkin's answer, you might be interested in these articles by Eric Lippert :

Do not name a class the same as its namespace, Part One
Do not name a class the same as its namespace, Part Two
Do not name a class the same as its namespace, Part Three
Do not name a class the same as its namespace, Part Four

They are not directly related to your problem, but they give an interesting insight on naming strategies

var x = new global::Foo.Class1();