Method invocation failed because [System.Management.Automation.PSObject] doesn't contain a method named 'op_Addition'

Question

I am trying to export some data from sharepoint list to csv and I got this error:

$ListItemCollection | Export-CSV \"D:\\LX.csv\" -NoTypeInformation

Answer 1

tl;dr:

• $ListItemCollection is of type [System.Management.Automation.PSObject], not an array.

• Make sure that it is an array (e.g., $ListItemCollection = @()) for += to work as intended, i.e., for += to append an element^[1].

• Note that commands that typically output multiple items - which are then collected in a regular [object[]] array, if assigned to a variable - output only a scalar if the command situationally happens to return only one item; in other words: a single-item output array is automatically unwrapped.

• Therefore, if there's a chance that a command situationally returns only a single object, yet you need the result to always be an array, use @(...), the array-subexpression operator; e.g.:

       # @(...) ensures that $file is an array, even if just 1 file matches
       $files = @(Get-ChildItem *.txt)
  

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The error message implies that $ListItemCollection is of type [System.Management.Automation.PSObject] and not an array.

Since type [pscustomobject] ([System.Management.Automation.PSObject]) does not have a static op_Addition method, you cannot use the + operator with an instance of it as the LHS.
(Type-specific operators are implemented as static op_* methods).

You can verify this as follows:

PS> (New-Object System.Management.Automation.PSObject) + 1 # !! Breaks
Method invocation failed because [System.Management.Automation.PSObject] does not contain a method named 'op_Addition'

If you want to inspect a given type for operator support, use a command such as the following, using the [datetime] type as an example:

PS> [datetime] | Get-Member -Force -Static -Type Method op_*

   TypeName: System.DateTime

Name                  MemberType Definition
----                  ---------- ----------
op_Addition           Method     static datetime op_Addition(datetime d, timespan t)
op_Equality           Method     static bool op_Equality(datetime d1, datetime d2)
op_GreaterThan        Method     static bool op_GreaterThan(datetime t1, datetime t2)
op_GreaterThanOrEqual Method     static bool op_GreaterThanOrEqual(datetime t1, datetime t2)
op_Inequality         Method     static bool op_Inequality(datetime d1, datetime d2)
op_LessThan           Method     static bool op_LessThan(datetime t1, datetime t2)
op_LessThanOrEqual    Method     static bool op_LessThanOrEqual(datetime t1, datetime t2)
op_Subtraction        Method     static datetime op_Subtraction(datetime d, timespan t), static timespan op_Subtraction(datetime d1, datetime d2)

Note:

• The "primitive" .NET data types do not have such methods, because operator support for them is built in.

• Similarly, it is PowerShell itself that implements + for arrays and collections ([object[]], [System.Collections.Generic.List[object]], ...), though note that:

  1. a new instance is invariably constructed, and
  2. the result is always of type [object[]] (unless you use a type-constrained variable that converts the array back to a different collection type).

• -Force is needed, because Get-Member hides the op_* methods by default.

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^{[1] Technically, a new array is created behind the scenes, because arrays are immutable. In loops this can be a performance concern; if so, use a mutable collection type such as [System.Collections.Generic.List[object]] and append to it with its .Add() method.}