How to pass a parameter from C# to a PowerShell script file?

How's this?

static void Main()
{
    string script = @"C:\test.ps1 -arg 'hello world!'";
    StringBuilder sb = new StringBuilder();

    PowerShell psExec = PowerShell.Create();
    psExec.AddScript(script);
    psExec.AddCommand("out-string");

    Collection<PSObject> results;
    Collection<ErrorRecord> errors;
    results = psExec.Invoke();
    errors = psExec.Streams.Error.ReadAll();

    if (errors.Count > 0)
    {
        foreach (ErrorRecord error in errors)
        {
            sb.AppendLine(error.ToString());
        }
    }
    else
    {
        foreach (PSObject result in results)
        {
            sb.AppendLine(result.ToString());
        }
    }

    Console.WriteLine(sb.ToString());
}

Here's a similar version that passes an instance of a DateTime

static void Main()
{
    StringBuilder sb = new StringBuilder();

    PowerShell psExec = PowerShell.Create();
    psExec.AddCommand(@"C:\Users\d92495j\Desktop\test.ps1");
    psExec.AddArgument(DateTime.Now);

    Collection<PSObject> results;
    Collection<ErrorRecord> errors;
    results = psExec.Invoke();
    errors = psExec.Streams.Error.ReadAll();

    if (errors.Count > 0)
    {
        foreach (ErrorRecord error in errors)
        {
            sb.AppendLine(error.ToString());
        }
    }
    else
    {
        foreach (PSObject result in results)
        {
            sb.AppendLine(result.ToString());
        }
    }

    Console.WriteLine(sb.ToString());
}

So, to make a long answer short: Use AddCommand instead of AddScript

another way is to fill the runspace with variables.

    public static string RunPs1File(string filePath, Dictionary<string, object> arguments)
    {
        var result = new StringBuilder();

        using (Runspace space = RunspaceFactory.CreateRunspace())
        {
            space.Open();

            foreach (KeyValuePair<string, object> variable in arguments)
            {
                var key = new string(variable.Key.Where(char.IsLetterOrDigit).ToArray());
                space.SessionStateProxy.SetVariable(key, variable.Value);
            }

            string script = System.IO.File.ReadAllText(filePath);
            using (PowerShell ps1 = PowerShell.Create())
            {
                ps1.Runspace = space;
                ps1.AddScript(script);

                var psOutput = ps1.Invoke();

                var errors = ps1.Streams.Error;
                if (errors.Count > 0)
                {
                    var e = errors[0].Exception;
                    ps1.Streams.ClearStreams();
                    throw e;
                }

                foreach (var line in psOutput)
                {
                    if (line != null)
                    {
                        result.AppendLine(line.ToString());
                    }
                }
            }
        }

        return result.ToString();
    }