Can a player character created with the build rules of D&D 3.5e be played in 5e without conversion? Or is the difference between the editions too much?
No - converting a character from the 3.5 ruleset to 5e requires substantial translation work.
The rulesets for 3.5 and 5e are simply too different to take a 3.5 character and play them in 5e without changes:
- The games have different classes, and different rules for combining them. For instance, the Truenamer class (from 3.5) doesn't exist in 5e. What are you supposed to do if your character has levels in a class that doesn't exist in 5e?
- Even for classes that exist in both editions, they provide different benefits. For instance, in 3.5, the Fighter class provides bonus feats as its primary benefit. In 5e, on the other hand, feats aren't even assumed to exist; they're a variant rule. If your table isn't using the rule, what are you supposed to do with all those bonus feats your character has?
- The games run on vastly different assumptions about what characters are capable of at a given level. For instance, 5e has bounded accuracy as a core design concept. 3.5 didn't, so directly taking the numbers from a 3.5 character and running them without changes in 5e would result in a character capable of hitting numbers much, much higher than 5e enemies are designed to cope with.
- The basic rules of defenses changed between 3.5 and 5e. For instance, a 5e character has a saving throw associated with each ability score, but a 3.5 character only had Reflex, Fortitude, and Will (associated with Dexterity, Constitution, and Wisdom, respectively). When your 3.5 character is called upon to make a Charisma save, what number are they supposed to use? Charisma saves didn't exist when they were created!
- There are countless other examples of changes in ruleset that make direct transfer impossible. For instance, there are plenty of spells that exist in 3.5 but not in 5e. What are you supposed to do when your 3.5 character knows one of these spells and wants to cast it in a 5e game?
You can read up on the (many!) differences between the editions in the answers here. These differences are sufficiently enormous that in response to a related question about converting from Pathfinder (very similar to the 3.5 ruleset) to 5e, the highest-voted answer is just, "find similar-sounding races and classes, then use them to make a new character from scratch."