RequiredAttribute Class

项目
05/05/2015

Defines the metadata attribute that task authors use to identify required task properties. Task properties with this attribute must have a set value when the task is run.

MSBuild is now included in Visual Studio instead of the .NET Framework. You can use MSBuild 12.0 side-by-side with versions previously deployed with the .NET Framework.For more information, see What's New in MSBuild 12.0.

Inheritance Hierarchy

System.Object
System.Attribute
Microsoft.Build.Framework.RequiredAttribute

Namespace: Microsoft.Build.Framework
Assembly: Microsoft.Build.Framework (in Microsoft.Build.Framework.dll)

Syntax

'Declaration
<AttributeUsageAttribute(AttributeTargets.Property, AllowMultiple := False,  _
    Inherited := False)> _
Public NotInheritable Class RequiredAttribute _
    Inherits Attribute

[AttributeUsageAttribute(AttributeTargets.Property, AllowMultiple = false, 
    Inherited = false)]
public sealed class RequiredAttribute : Attribute

[AttributeUsageAttribute(AttributeTargets::Property, AllowMultiple = false, 
    Inherited = false)]
public ref class RequiredAttribute sealed : public Attribute

[<Sealed>]
[<AttributeUsageAttribute(AttributeTargets.Property, AllowMultiple = false, 
    Inherited = false)>]
type RequiredAttribute =  
    class 
        inherit Attribute 
    end

public final class RequiredAttribute extends Attribute

The RequiredAttribute type exposes the following members.

Constructors

	Name	Description
	RequiredAttribute	Initializes a new instance of the RequiredAttribute class.MSBuild is now included in Visual Studio instead of the .NET Framework. You can use MSBuild 12.0 side-by-side with versions previously deployed with the .NET Framework.For more information, see What's New in MSBuild 12.0.

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Properties

	Name	Description
	TypeId	(Inherited from Attribute.)

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Methods

	Name	Description
	Equals	(Inherited from Attribute.)
	GetHashCode	(Inherited from Attribute.)
	GetType	(Inherited from Object.)
	IsDefaultAttribute	(Inherited from Attribute.)
	Match	(Inherited from Attribute.)
	ToString	(Inherited from Object.)

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Explicit Interface Implementations

	Name	Description
	System#Runtime#InteropServices#_Attribute#GetIDsOfNames	(Inherited from Attribute.)
	System#Runtime#InteropServices#_Attribute#GetTypeInfo	(Inherited from Attribute.)
	System#Runtime#InteropServices#_Attribute#GetTypeInfoCount	(Inherited from Attribute.)
	System#Runtime#InteropServices#_Attribute#Invoke	(Inherited from Attribute.)

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Remarks

If a property marked with this attribute is not assigned a value when the task is invoked, the build will fail.

For more information about using attributes, see Extending Metadata Using Attributes.

Examples

The following example shows the code for a task that creates one or more directories.

using System;
using System.IO;
using System.Security;
using System.Collections;
using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace Microsoft.Build.Tasks
{
    /*
     * Class: MakeDir
     * An MSBuild task that creates one or more directories.
     */ 
    public class MakeDir : Task
    {
        // The Required attribute indicates the following to MSBuild: 
        //       - if the parameter is a scalar type, and it is not supplied, fail the build immediately 
        //       - if the parameter is an array type, and it is not supplied, pass in an empty array 
        // In this case the parameter is an array type, so if a project fails to pass in a value for the  
            // Directories parameter, the task will get invoked, but this implementation will do nothing, 
            // because the array will be empty.
        [Required]
            // Directories to create. 
        public ITaskItem[] Directories
        {
            get
            {
                return directories;
            }

            set
            {
                directories = value;
            }
        }

        // The Output attribute indicates to MSBuild that the value of this property can be gathered after the 
        // task has returned from Execute(), if the project has an <Output> tag under this task's element for  
        // this property.
        [Output]
        // A project may need the subset of the inputs that were actually created, so make that available here. 
        public ITaskItem[] DirectoriesCreated
        {
            get
            {
                return directoriesCreated;
            }
        }

        private ITaskItem[] directories;
        private ITaskItem[] directoriesCreated;

        /// <summary> 
        /// Execute is part of the Microsoft.Build.Framework.ITask interface. 
        /// When it's called, any input parameters have already been set on the task's properties. 
        /// It returns true or false to indicate success or failure. 
        /// </summary> 
        public override bool Execute()
        {
            ArrayList items = new ArrayList();
            foreach (ITaskItem directory in Directories)
            {
                // ItemSpec holds the filename or path of an Item 
                if (directory.ItemSpec.Length > 0)
                {
                    try
                    {
                        // Only log a message if we actually need to create the folder 
                        if (!Directory.Exists(directory.ItemSpec))
                        {
                            Log.LogMessage(MessageImportance.Normal, "Creating directory " + directory.ItemSpec);
                            Directory.CreateDirectory(directory.ItemSpec);
                        }

                        // Add to the list of created directories
                        items.Add(directory);
                    }
                    // If a directory fails to get created, log an error, but proceed with the remaining  
                    // directories. 
                    catch (Exception ex)
                    {
                        if (ex is IOException
                            || ex is UnauthorizedAccessException
                            || ex is PathTooLongException
                            || ex is DirectoryNotFoundException
                            || ex is SecurityException)
                        {
                            Log.LogError("Error trying to create directory " + directory.ItemSpec + ". " + ex.Message);
                        }
                        else
                        {
                            throw;
                        }
                    }
                }
            }

            // Populate the "DirectoriesCreated" output items.
            directoriesCreated = (ITaskItem[])items.ToArray(typeof(ITaskItem));

            // Log.HasLoggedErrors is true if the task logged any errors -- even if they were logged  
            // from a task's constructor or property setter. As long as this task is written to always log an error 
            // when it fails, we can reliably return HasLoggedErrors. 
            return !Log.HasLoggedErrors;
        }
    }
}

To run this task use a project file like this:

<Project xmlns="https://schemas.microsoft.com/developer/msbuild/2003">
    <Target Name="MyTarget">
        <MkDir Directories="NewDir" />
    </Target>
</Project>

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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