SSIS Connection String Expression at Run Time and during Execution I have an Excel connection manager which should write to a file which does not exist prior to runtime but rather is created during execution. I have an expression set for the connection string property of the excel connection manager which points to the file that will be created. The problem I am having is that since the file does not yet exist (prior to execution), when I go to run the package it is giving me an error since it can not find the file. If i create a dummy file and place it where the excel connec manager is pointing the package will run ok. 

What I have tried to do is point the excel connection manager to an existing file prior to run time since it will not give an error and then hopping it will then pick up the new Path (excel connection string) from the Expressions but what is happening is that since I have an expressions set for the connection string property it will not let me overwrite the connection string property, it keeps defaulting to the value of the expressions which contains other variable that are populated during run time so that is where I am getting my error. The expressions string prior to runtime is not pointing to an actual file because it does not yet exist but mainly because the expressions is not fully populated until runtime since it also contains variables.

As of right now it seems like I might have to just point to a dummy template file (without using any expressions) prior to runtime and then maybe use a Script to change the connection string property of the excel connec manager to = a variable containing an expression containing the path to the file....

Does this sound like the way to go  or am I over complicating things?
When a package receives a signal to start, one of the tasks it performs is to ensure everything it needs is available and has the appropriate signatures. SSIS is all about the metadata. The idea behind this is that a package should blow up before it does any work if it can tell at the start that an error will occur (can't connect to a server, table doesn't exist, data type is incompatible, etc). It's generally a good thing, why start something you know you can't finish and potentially leave yourself in a messy/inconsistent state.

In your scenario, it is validating that this file is there and since it isn't, because it's generated at run-time, it fails. Therefore, you have to flag it such that the validation occurs right at the moment the resource is needed. As @mike Henderson indicated, you'll need to go into the properties of the objects and change the DelayValidation value from False to True. You will probably need to set it on both the Connection Manager and the Data Flow Task that uses it. Click once on the item, hit F4 or right click and select Properties.