Strongly Encouraged Development Guidelines
This section describes guidelines that you should follow when you write your cmdlets. They are separated into guidelines for designing cmdlets and guidelines for writing your cmdlet code. You might find that these guidelines are not applicable for every scenario. However, if they do apply and you do not follow these guidelines, your users might have a poor experience when they use your cmdlets.
The following guidelines should be followed when designing cmdlets to ensure a consistent user experience between using your cmdlets and other cmdlets. When you find a Design guideline that applies to your situation, be sure to look at the Code guidelines for similar guidelines.
Nouns used in cmdlet naming need to be very specific so that the user can discover your cmdlets. Prefix generic nouns such as "server" with a shortened version of the product name. For example, if a noun refers to a server that is running an instance of Microsoft SQL Server, use a noun such as "SQLServer". The combination of specific nouns and the short list of approved verbs enable the user to quickly discover and anticipate functionality while avoiding duplication among cmdlet names.
To enhance the user experience, the noun that you choose for a cmdlet name should be singular. For example, use the name Get-Process instead of Get-Processes. It is best to follow this rule for all cmdlet names, even when a cmdlet is likely to act upon more than one item.
Use Pascal case for parameter names. In other words, capitalize the first letter of verb and all terms used in the noun. For example, “Clear-ItemProperty”.
A cmdlet needs parameters that receive the data on which it must operate, and parameters that indicate information that is used to determine the characteristics of the operation. For example, a cmdlet might have a Name parameter that receives data from the pipeline, and the cmdlet might have a Force parameter to indicate that the cmdlet can be forced to perform its operation. There is no limit to the number of parameters that a cmdlet can define.
Your cmdlet should use standard parameter names so that the user can quickly determine what a particular parameter means. If a more specific name is required, use a standard parameter name, and then specify a more specific name as an alias. For example, the Get-Service cmdlet has a parameter that has a generic name (Name) and a more specific alias (ServiceName). Both terms can be used to specify the parameter.
For more information about parameter names and their data types, see Cmdlet Parameter Name and Functionality Guidelines.
Avoid using plural names for parameters whose value is a single element. This includes parameters that take arrays or lists because the user might supply an array or list with only one element.
Plural parameter names should be used only in those cases where the value of the parameter is always a multiple-element value. In these cases, the cmdlet should verify that multiple elements are supplied, and the cmdlet should display a warning to the user if multiple elements are not supplied.
Use Pascal case for parameter names. In other words, capitalize the first letter of each word in the parameter name, including the first letter of the name. For example, the parameter name ErrorAction uses the correct capitalization. The following parameter names use incorrect capitalization:
There are two ways to create a parameter whose value can be selected from a set of options.
Define an enumeration type (or use an existing enumeration type) that specifies the valid values. Then, use the enumeration type to create a parameter of that type.
Add the ValidateSet attribute to the parameter declaration. For more information about this attribute, see ValidateSet Attribute Declaration.
To ensure consistency with other cmdlets, use standard types for parameters where ever possible. For more information about the types that should be used for different parameter, see Standard Cmdlet Parameter Names and Types. This topic provides links to several topics that describe the names and .NET Framework types for groups of standard parameters, such as the “activity parameters”.
Parameters should be defined as .NET Framework types to provide better parameter validation. For example, parameters that are restricted to one value from a set of values should be defined as an enumeration type. To support a Uniform Resource Identifier (URI) value, define the parameter as a Uri type. Avoid basic string parameters for all but free-form text properties.
If your parameter takes only true and false, define the parameter as type SwitchParameter. A switch parameter is treated as true when it is specified in a command. If the parameter is not included in a command, Windows PowerShell considers the value of the parameter to be false. Do not define Boolean parameters.
If your parameter needs to differentiate between 3 values: $true, $false and “unspecified”, then define a parameter of type Nullable<bool>. The need for a 3rd, "unspecified" value typically occurs when the cmdlet can modify a Boolean property of an object. In this case "unspecified" means to not change the current value of the property.
Frequently, users must perform the same operation against multiple arguments. For these users, a cmdlet should accept an array as parameter input so that a user can pass the arguments into the parameter as a Windows PowerShell variable. For example, the Get-Process cmdlet uses an array for the strings that identify the names of the processes to retrieve.
By default, many cmdlets that modify the system, such as the Stop-Process cmdlet, act as "sinks" for objects and do not return a result. These cmdlet should implement the PassThru parameter to force the cmdlet to return an object. When the PassThru parameter is specified, the cmdlet returns an object by using a call to the WriteObject method. For example, the following command stops the Calc process and passes the resultant process to the pipeline.
stop-process calc –passthru
In most cases, Add, Set, and New cmdlets should support a PassThru parameter.
A cmdlet is intended to accomplish a single purpose. However, there is frequently more than one way to describe the operation or the operation target. For example, a process might be identified by its name, by its identifier, or by a process object. The cmdlet should support all the reasonable representations of its targets. Normally, the cmdlet satisfies this requirement by specifying sets of parameters (referred to as parameter sets) that operate together. A single parameter can belong to any number of parameter sets. For more information about parameter sets, see Cmdlet Parameter Sets.
When you specify parameter sets, set only one parameter in the set to ValueFromPipeline. For more information about how to declare the Parameter attribute, see ParameterAttribute Declaration.
When parameter sets are used, the default parameter set is defined by the Cmdlet attribute. The default parameter set should include the parameters most likely to be used in an interactive Windows PowerShell session. For more information about how to declare the Cmdlet attribute, see CmdletAttribute Declaration.
Use the guidelines in this section to provide feedback to the user. This feedback allows the user to be aware of what is occurring in the system and to make better administrative decisions.
The Windows PowerShell runtime allows a user to specify how to handle output from each call to the Write method by setting a preference variable. The user can set several preference variables, including a variable that determines whether the system should display information and a variable that determines whether the system should query the user before taking further action.
A cmdlet should call the WriteWarning method when the cmdlet is about to perform an operation that might have an unintended result. For example, a cmdlet should call this method if the cmdlet is about to overwrite a read-only file.
A cmdlet should call the WriteVerbose method when the user requires some detail about what the cmdlet is doing. For example, a cmdlet should call this information if the cmdlet author feels that there are scenarios that might require more information about what the cmdlet is doing.
The cmdlet should call the WriteDebug method when a developer or product support engineer must understand what has corrupted the cmdlet operation. It is not necessary for the cmdlet to call the WriteDebug method in the same code that calls the WriteVerbose method because the Debug parameter presents both sets of information.
Cmdlet operations that take a long time to complete and that cannot run in the background should support progress reporting through periodic calls to the WriteProgress method.
Occasionally, a cmdlet must communicate directly with the user instead of by using the various Write or Should methods supported by the Cmdlet class. In this case, the cmdlet should derive from the PSCmdlet class and use the Host property. This property supports different levels of communication type, including the PromptForChoice, Prompt, and WriteLine/ReadLine types. At the most specific level, it also provides ways to read and write individual keys and to deal with buffers.
Unless a cmdlet is specifically designed to generate a graphical user interface (GUI), it should not bypass the host by using the Host property. An example of a cmdlet that is designed to generate a GUI is the Out-GridView cmdlet.
Cmdlets should not use the Console API.
For each cmdlet assembly, create a Help.xml file that contains information about the cmdlet. This information includes a description of the cmdlet, descriptions of the cmdlet's parameters, examples of the cmdlet's use, and more.
The following guidelines should be followed when coding cmdlets to ensure a consistent user experience between using your cmdlets and other cmdlets. When you find a Code guideline that applies to your situation, be sure to look at the Design guidelines for similar guidelines.
Define a parameter by declaring a public property of the cmdlet class that is decorated with the Parameter attribute. Parameters do not have to be static members of the derived .NET Framework class for the cmdlet. For more information about how to declare the Parameter attribute, see Parameter Attribute Declaration.
The Windows PowerShell path is the mechanism for normalizing access to namespaces. When you assign a Windows PowerShell path to a parameter in the cmdlet, the user can define a custom "drive" that acts as a shortcut to a specific path. When a user designates such a drive, stored data, such as data in the Registry, can be used in a consistent way.
If your cmdlet allows the user to specify a file or a data source, it should define a parameter of type String. If more than one drive is supported, the type should be an array. The name of the parameter should be Path, with an alias of PSPath. Additionally, the Path parameter should support wildcard characters. If support for wildcard characters is not required, define a LiteralPath parameter.
If the data that the cmdlet reads or writes has to be a file, the cmdlet should accept Windows PowerShell path input, and the cmdlet should use the SessionState.Path property to translate the Windows PowerShell paths into paths that the file system recognizes. The specific mechanisms include the following methods:
If the data that the cmdlet reads or writes is only a set of strings instead of a file, the cmdlet should use the provider content information (Content member) to read and write. This information is obtained from the InvokeProvider property. These mechanisms allow other data stores to participate in the reading and writing of data.
A cmdlet should support wildcard characters if possible. Support for wildcard characters occurs in many places in a cmdlet (especially when a parameter takes a string to identify one object from a set of objects). For example, the sample Stop-Proc cmdlet from the StopProc Tutorial defines a Name parameter to handle strings that represent process names. This parameter supports wildcard characters so that the user can easily specify the processes to stop.
When support for wildcard characters is available, a cmdlet operation usually produces an array. Occasionally, it does not make sense to support an array because the user might use only a single item at a time. For example, the Set-Location cmdlet does not need to support an array because the user is setting only a single location. In this instance, the cmdlet still supports wildcard characters, but it forces resolution to a single location.
For more information about wildcard-character patterns, see Supporting Wildcard Characters in Cmdlet Parameters.
This section contains guidelines for defining objects for cmdlets and for extending existing objects.
Define standard members to extend an object type in a custom Types.ps1xml file (use the Windows PowerShell Types.ps1xml file as a template). Standard members are defined by a node with the name PSStandardMembers. These definitions allow other cmdlets and the Windows PowerShell runtime to work with your object in a consistent way.
If you are designing an object for a cmdlet, ensure that its members map directly to the parameters of the cmdlets that will use it. This mapping allows the object to be easily sent to the pipeline and to be passed from one cmdlet to another.
Preexisting .NET Framework objects that are returned by cmdlets are frequently missing some important or convenient members that are needed by the script developer or user. These missing members can be particularly important for display and for creating the correct member names so that the object can be correctly passed to the pipeline. Create a custom Types.ps1xml file to document these required members. When you create this file, we recommend the following naming convention: <Your_Product_Name>.Types.ps1xml.
For example, you could add a Mode script property to the FileInfo type to display the attributes of a file more clearly. Additionally, you could add a Count alias property to the Array type to allow the consistent use of that property name (instead of Length).
Implement a IComparable interface on all output objects. This allows the output objects to be easily piped to various sorting and analysis cmdlets.
If the display for an object does not provide the expected results, create a custom <YourProductName>.Format.ps1xml file for that object.
Implement a cmdlet assuming that it will be called from the middle of a pipeline (that is, other cmdlets will produce its input or consume its output). For example, you might assume that the Get-Process cmdlet, because it generates data, is used only as the first cmdlet in a pipeline. However, because this cmdlet is designed for the middle of a pipeline, this cmdlet allows previous cmdlets or data in the pipeline to specify the processes to retrieve.
In each parameter set for a cmdlet, include at least one parameter that supports input from the pipeline. Support for pipeline input allows the user to retrieve data or objects, to send them to the correct parameter set, and to pass the results directly to a cmdlet.
A parameter accepts input from the pipeline if the Parameter attribute includes the ValueFromPipeline keyword, the ValueFromPipelineByPropertyName keyword attribute, or both keywords in its declaration. If none of the parameters in a parameter set support the ValueFromPipeline or ValueFromPipelineByPropertyName keywords, the cmdlet cannot meaningfully be placed after another cmdlet because it will ignore any pipeline input.
When a cmdlet returns objects, the cmdlet should write the objects immediately as they are generated. The cmdlet should not hold them in order to buffer them into a combined array. The cmdlets that receive the objects as input will then be able to process, display, or process and display the output objects without delay. A cmdlet that generates output objects one at a time should call the Cmdlet.WriteObject method. A cmdlet that generates output objects in batches (for example, because an underlying API returns an array of output objects) should call the M:System.Managemet.Automation.Cmdlet.WriteObject(System.Object,System.Boolean) Method with its second parameter set to true.
By default, Windows PowerShell itself is case-insensitive. However, because it deals with many preexisting systems, Windows PowerShell does preserve case for ease of operation and compatibility. In other words, if a character is supplied in uppercase letters, Windows PowerShell keeps it in uppercase letters. For systems to work well, a cmdlet needs to follow this convention. If possible, it should operate in a case-insensitive way. It should, however, preserve the original case for cmdlets that occur later in a command or in the pipeline.