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In order to catch and handle errors in PowerShell we can use the Trap keyword. Trap has the following Syntax:

trap [[error type]] {statement list}

If we try to merge a value of the type System.Int32 with a value of type System.String an error occurs.

PS > [int]1 + [string]"Hello"

Cannot convert value "Hello" to type "System.Int32". Error: "Input string was not in
a correct format."
At line:1 char:9
+ [int]1 + <<<< [string]"Hello"
+ CategoryInfo : NotSpecified: (:) [], RuntimeException
+ FullyQualifiedErrorId : RuntimeException

We can get the type of error that occured by checking the InnerException.

PS > $Error[0].Exception.InnerException.GetType().FullName

System.Management.Automation.PSInvalidCastException

With this information, we can handle the errors of the type System.Management.Automation.PSInvalidCastException using a simple trap. In the example below
we create a funtion that shows how to handle these types of errors.

function TrapError([scriptblock]$Command) {
  Trap [System.Management.Automation.PSInvalidCastException] {
    Write-Host "My Custom Error Message" -ForegroundColor Red
    Continue
  }
  & $Command
}

When we call the function our custom error message is returned instead of the default error message.

PS > TrapError -Command { [int]1 + [string]"Hello" }

My Custom Error Message

But what if we generate another kind of error. Let's try to divide 1 by $null.

PS > TrapError -Command { 1/$null }

Attempted to divide by zero.
At line:1 char:24
+ TrapError -Command { 1/ <<<< $null }
    + CategoryInfo          : NotSpecified: (:) [], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : RuntimeException

This time the error was not handled since it's a different type of error. We can of course modify our function to handle these types of errors as well.

function TrapError([scriptblock]$Command) {
  Trap [System.Management.Automation.PSInvalidCastException] {
    Write-Host "My Custom Error Message" -ForegroundColor Red
    Continue
  }

  Trap [System.DivideByZeroException] {
    Write-Host "1/`$null doesn't really work.." -ForegroundColor Red
    Continue
  }

  & $Command
}

If we try the same command again, the error is handled.

PS > TrapError -Command { 1/$null }

1/$null doesn't really work..

We can also add a Trap keyword without a specific error type to handle other types of errors that may occur.

function TrapError([scriptblock]$Command) {
  Trap [System.Management.Automation.PSInvalidCastException] {
    Write-Host "My Custom Error Message" -ForegroundColor Red
    Continue
  }

  Trap [System.DivideByZeroException] {
    Write-Host "1/`$null doesn't really work.." -ForegroundColor Red
    Continue
  }

  Trap {
    Write-Host "This really doesn't work.." -ForegroundColor Red
    Continue
  }

  & $Command
}

If we try to create an instance of a .NET Object and specify a nonexisting class, the error is handled by the trap.

PS > TrapError -Command { New-Object System.Blah }

This really doesn't work..

Some Errors are handled in other ways in commands and scripts. a typical example is the Get-ChildItem cmdlet.
If we use our function and use Get-ChildItem and try to retrieve a folder that doesn't exist, the cmdlet will handle
the error.

PS > TrapError -Command { Get-ChildItem C:\Blah }

Get-ChildItem : Cannot find path 'C:\Blah' because it does not exist.
At line:1 char:35
+ TrapError -Command { Get-ChildItem <<<<  C:\Blah }
    + CategoryInfo          : ObjectNotFound: (C:\Blah:String) [Get-ChildItem], ItemNotFoundException
    + FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.GetChildItemCommand

We can override this and manage the error in our trap by setting the ErrorAction parameter to Stop.

PS > TrapError -Command { Get-ChildItem C:\Blah -ErrorAction Stop }

This really doesn't work..