functions/New-DbaDbUser.ps1

function New-DbaDbUser {
    <#
    .SYNOPSIS
        Creates a new user for the specified database.
 
    .DESCRIPTION
        Creates a new user for a specified database with provided specifications.
 
    .PARAMETER SqlInstance
        The target SQL Server instance or instances. Defaults to the default instance on localhost.
 
    .PARAMETER SqlCredential
        Login to the target instance using alternative credentials. Accepts PowerShell credentials (Get-Credential).
 
        Windows Authentication, SQL Server Authentication, Active Directory - Password, and Active Directory - Integrated are all supported.
 
        For MFA support, please use Connect-DbaInstance.
 
    .PARAMETER Database
        Specifies the database(s) to process. Options for this list are auto-populated from the server. If unspecified, all databases will be processed.
 
    .PARAMETER ExcludeDatabase
        Specifies the database(s) to exclude from processing. Options for this list are auto-populated from the server. By default, system databases are excluded.
 
    .PARAMETER IncludeSystem
        If this switch is enabled, the user will be added to system databases.
 
    .PARAMETER Login
        When specified, the user will be associated to this SQL login and have the same name as the Login.
 
    .PARAMETER Username
        When specified, the user will have this name.
 
    .PARAMETER Force
        If user exists, drop and recreate.
 
    .PARAMETER WhatIf
        If this switch is enabled, no actions are performed but informational messages will be displayed that explain what would happen if the command were to run.
 
    .PARAMETER Confirm
        If this switch is enabled, you will be prompted for confirmation before executing any operations that change state.
 
    .PARAMETER EnableException
        By default, when something goes wrong we try to catch it, interpret it and give you a friendly warning message.
        This avoids overwhelming you with "sea of red" exceptions, but is inconvenient because it basically disables advanced scripting.
        Using this switch turns this "nice by default" feature off and enables you to catch exceptions with your own try/catch.
 
    .NOTES
        Tags: Database, User
        Author: Frank Henninger (@osiris687)
 
        Website: https://dbatools.io
        Copyright: (c) 2018 by dbatools, licensed under MIT
        License: MIT https://opensource.org/licenses/MIT
 
    .LINK
        https://dbatools.io/New-DbaDbUser
 
    .EXAMPLE
        PS C:\> New-DbaDbUser -SqlInstance sqlserver2014 -Database DB1 -Login user1
 
        Creates a new sql user with login named user1 in the specified database.
 
    .EXAMPLE
        PS C:\> New-DbaDbUser -SqlInstance sqlserver2014 -Database DB1 -Username user1
 
        Creates a new sql user without login named user1 in the specified database.
 
    .EXAMPLE
        PS C:\> New-DbaDbUser -SqlInstance sqlserver2014 -Database DB1 -Login Login1 -Username user1
 
        Creates a new sql user named user1 mapped to Login1 in the specified database.
 
    .EXAMPLE
        PS C:\> Get-DbaDbUser -SqlInstance sqlserver1 -Database DB1 | New-DbaDbUser -SqlInstance sqlserver2 -Database DB1
 
        Copies users from sqlserver1.DB1 to sqlserver2.DB1. Does not copy permissions!
 
    #>

    [CmdletBinding(SupportsShouldProcess, DefaultParameterSetName = "NoLogin", ConfirmImpact = "Medium")]
    param(
        [parameter(Mandatory, Position = 1)]
        [DbaInstanceParameter[]]$SqlInstance,
        [PSCredential]$SqlCredential,
        [object[]]$Database,
        [object[]]$ExcludeDatabase,
        [switch]$IncludeSystem,
        [parameter(ParameterSetName = "Login")]
        [string]$Login,
        [parameter(ParameterSetName = "NoLogin")]
        [parameter(ParameterSetName = "Login")]
        [string[]]$Username,
        [switch]$Force,
        [switch]$EnableException
    )

    begin {
        if ($Force) { $ConfirmPreference = 'none' }

        function Test-SqlLoginInDatabase {
            param(
                [Microsoft.SqlServer.Management.Smo.Login]$Login,
                [Microsoft.SqlServer.Management.Smo.Database]$Database
            )

            # Does user exist with same login?
            if ( $existingUser = ( $Database.Users | Where-Object Login -eq $smoLogin ) ) {
                if ($Force) {
                    if ($Pscmdlet.ShouldProcess($existingUser, "Dropping existing user $($existingUser.Name) because -Force was used")) {
                        try {
                            $existingUser.Drop()
                        } catch {
                            Stop-Function -Message "Could not remove existing user $($existingUser.Name), skipping." -Target $existingUser -ErrorRecord $_ -Exception $_.Exception.InnerException.InnerException.InnerException -Continue
                        }
                    }
                } else {
                    Stop-Function -Message "User $($existingUser.Name) already exists and -Force was not specified" -Target $existingUser -Continue
                }
            }
        }

        function Test-SqlUserInDatabase {
            param(
                [string[]]$Username,
                [Microsoft.SqlServer.Management.Smo.Database]$Database
            )

            # Does user exist with same login?
            if ( $existingUser = ( $Database.Users | Where-Object Name -eq $Username ) ) {
                if ($Force) {
                    if ($Pscmdlet.ShouldProcess($existingUser, "Dropping existing user $($existingUser.Name) because -Force was used")) {
                        try {
                            $existingUser.Drop()
                        } catch {
                            Stop-Function -Message "Could not remove existing user $($existingUser.Name), skipping." -Target $existingUser -ErrorRecord $_ -Exception $_.Exception.InnerException.InnerException.InnerException -Continue
                        }
                    }
                } else {
                    Stop-Function -Message "User $($existingUser.Name) already exists and -Force was not specified" -Target $existingUser -Continue
                }
            }
        }
    }

    process {
        foreach ($instance in $SqlInstance) {
            try {
                $server = Connect-SqlInstance -SqlInstance $instance -SqlCredential $SqlCredential
            } catch {
                Stop-Function -Message "Error occurred while establishing connection to $instance" -Category ConnectionError -ErrorRecord $_ -Target $instance -Continue
            }

            $databases = $server.Databases | Where-Object IsAccessible -eq $true

            if ($Database) {
                $databases = $databases | Where-Object Name -In $Database
            }
            if ($ExcludeDatabase) {
                $databases = $databases | Where-Object Name -NotIn $ExcludeDatabase
            }
            if (Test-Bound 'IncludeSystem' -Not) {
                $databases = $databases | Where-Object IsSystemObject -NE $true
            }

            if ($null -eq $databases -or $databases.Count -eq 0) {
                Stop-Function -Message "Error occurred while establishing a connection to $Database" -Category ConnectionError -ErrorRecord $_ -Target $instance -Continue
            }

            foreach ($db in $databases) {
                Write-Message -Level Verbose -Message "Add users to Database $db on target $server"

                switch -Wildcard ($PSCmdlet.ParameterSetName) {
                    "Login*" {
                        # Creates a user with Login
                        Write-Message -Level VeryVerbose -Message "Using UserType: SqlLogin"

                        if ($PSBoundParameters.Keys -notcontains 'Login') {
                            Stop-Function -Message "Parameter -Login is required " -Target $instance
                        }
                        if ($Login.GetType().Name -eq 'Login') {
                            $smoLogin = $Login
                        } else {
                            #get the login associated with the given name.
                            $smoLogin = $server.Logins | Where-Object Name -eq $Login
                            if ($null -eq $smoLogin) {
                                Stop-Function -Message "Invalid Login: $Login is not found on $Server" -Target $instance;
                                return
                            }
                        }

                        Test-SqlLoginInDatabase -Database $db -Login $smoLogin

                        if ( $PSCmdlet.ParameterSetName -eq "LoginWithNewUsername" ) {
                            $Name = $Username
                            Write-Message -Level Verbose -Message "Using UserName: $Username"
                        } else {
                            $Name = $smoLogin.Name
                            Write-Message -Level Verbose -Message "Using LoginName: $Name"
                        }

                        $UserType = [Microsoft.SqlServer.Management.Smo.UserType]::SqlLogin
                    }

                    "NoLogin" {
                        # Creates a user without login
                        Write-Message -Level Verbose -Message "Using UserType: NoLogin"
                        $UserType = [Microsoft.SqlServer.Management.Smo.UserType]::NoLogin
                        $Name = $Username
                    }
                } #switch

                # Does user exist with same name?
                Test-SqlUserInDatabase -Database $db -Username $Name

                if ($Pscmdlet.ShouldProcess($db, "Creating user $Name")) {
                    try {
                        $smoUser = New-Object Microsoft.SqlServer.Management.Smo.User
                        $smoUser.Parent = $db
                        $smoUser.Name = $Name

                        if ( $PSBoundParameters.Keys -contains 'Login' -and $Login.GetType().Name -eq 'Login' ) {
                            $smoUser.Login = $Login
                        }
                        $smoUser.UserType = $UserType

                        $smoUser.Create()
                    } catch {
                        Stop-Function -Message "Failed to add user $Name in $db to $instance"  -Category InvalidOperation -ErrorRecord $_ -Target $instance -Continue
                    }
                    $smoUser.Refresh()

                    if ( $PSBoundParameters.Keys -contains 'Username' -and $smoUser.Name -ne $Username ) {
                        $smoUser.Rename($Username)
                    }

                    Write-Message -Level Verbose -Message "Successfully added $smoUser in $db to $instance."
                }

                #Display Results
                Get-DbaDbUser -SqlInstance $instance -SqlCredential $SqlCredential -Database $db.Name | Where-Object name -eq $smoUser.Name
            }
        }
    }
}