msbuild, defining Conditional Compilation Symbols

I'm possibly just blind, but is there a command line to specify conditional compilation symbols in MSBUILD?

I currently have this Line in my buildscript:

SET MSBUILD=C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\MSBuild.exe
SET CONFIG=Debug
%MSBUILD% /p:Configuration=%CONFIG% /p:OutputPath=..\..\output source\MyProject\MyProject.csproj

And I'd like to add a condition. In Visual Studio, i can just go into Project Properties => Build => Conditional compilation symbols, but I have not seen that option for msbuild?

Bonus Karma if you know if I can completely override all symbols already specified in the .csproj files to make sure that only the conditionals from my Buildscript go in.

Have you seen this? (most info is in the penultimate post)

/p:DefineConstants="MYSYMBOL1;MYSYMBOL2"

I had to use a space instead of a semicolon a la this post by Björn Lasar: http://www.linqinpark.net/2009/01/13/MSBuildWithMultipleDefineConstants.aspx

Update: the blog has disappeared; retrieved via Internet Archive:

> Recently I had to use MSBuild directly to automate some builds. I also had to configure some preprocessor defines based upon a configuration. This is usually done by an Argument like this

"/p:DefineConstants=MY_PREPROC_FLAG"

> Nothing special here since there are enough comments on the web about that. Today I needed one Flag more and I used the commandline syntax similar to how I knew it from the IDE:

"/p:DefineConstants=MY_PREPROC_FLAG;YET_ANOTHER_FLAG"

> but this one didn't work.

>So the point is that if you want to support multiple defines to a project by commandline you'll have to separate them by simple spaces...

"/p:DefineConstants=MY_PREPROC_FLAG YET_ANOTHER_FLAG" 

>and it will be added to the (semicolon-separated) Defines from the IDE. Good to know I think...

/p:DefineConstants is an all or nothing deal.

If you just want to turn off trace symbol, you can't just do it with: msbuild /p:DefineTrace=false

You have to define something to override all the symbols already defined: msbuild /p:DefineConstants="RANDOM-SYMBOL"

Thanks Michael Stum point this hidden rule out I have also wrote a blog about it --- dead link

What is said in the answers is valid for C# code, and also for ASP.NET "codebehind" C# code. For ASP.NET web projects, if you want to do conditional compilation in the ASPX pages as well, it works a bit differently to conditionally render HTML on the page (note I've removed MasterPageFile="..." AutoEventWireup="true" CodeBehind="..." Inherits="..." which you usually have in the <%@ ... %> declaration as well):

<%@ Page Title="MyPage" Language="C#" CompilerOptions="/d:DebugSym1;DebugSym2" %>

<% #if DebugSym1 %>         
	<h4>Section1</h4>
<% #else %>
	<h4>(Section 1 skipped)</h4>
<% #endif %>

<% #if DebugSym2 %>         
	<h4>Section2</h4>
<% #else %>
	<h4>(Section 2 skipped)</h4>
<% #endif %>

If you remove DebugSym1 or DebugSym2 from the CompilerOptions, then the #else part of the relevant #if statement is rendered.

I thought this was worth mentioning for completeness of this topic and can save you time. More you can find in this article, if you're interested.