Então, ambosisto eisto são bem claros. Simplesmente passe/p:DefineConstants="SYMBOL"

Não funciona nada para mim, mesmo em um projeto de teste. Eu estou esperando que passing / p: DefineConstants = "SYMBOL" irá sobrescrever quaisquer constantes de compilação condicional definidas no csproj. Não é o caso, no entanto ...

Listagem completa de código abaixo:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DefineConstants
{
    class Program
    {
        static void Main(string[] args)
        {
#if DEV
            Console.WriteLine("DEV");
#elif UAT 
            Console.WriteLine("UAT");
#else
            Console.WriteLine("No environment provided");
#endif
        }
    }
}

O arquivo .csproj é:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">x86</Platform>
    <ProductVersion>8.0.30703</ProductVersion>
    <SchemaVersion>2.0</SchemaVersion>
    <ProjectGuid>{57A2E870-0547-475C-B0EB-66CF9A2FE417}</ProjectGuid>
    <OutputType>Exe</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>DefineConstants</RootNamespace>
    <AssemblyName>DefineConstants</AssemblyName>
    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|x86' ">
    <PlatformTarget>x86</PlatformTarget>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>TRACE;DEBUG</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|x86' ">
    <PlatformTarget>x86</PlatformTarget>
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="System.Xml.Linq" />
    <Reference Include="System.Data.DataSetExtensions" />
    <Reference Include="System.Data" />
    <Reference Include="System.Xml" />
  </ItemGroup>
  <ItemGroup>
    <Compile Include="Program.cs" />
    <Compile Include="Properties\AssemblyInfo.cs" />
  </ItemGroup>
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
       Other similar extension points exist, see Microsoft.Common.targets.
  <Target Name="BeforeBuild">
  </Target>
  <Target Name="AfterBuild">
  </Target>
  -->
</Project>

construído usando:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\msbuild DefineConstants.sln /p:DefineConstants="DEV;DEBUG" /p:Configuration="Debug" /p:Platform="x86"

Executando o programa mostra:

No environment provided

Socorro!