c# - DrawUserPrimitives 无效操作异常

Question

我试图在 XNA 中使用这段代码绘制一个三角形:

VertexPositionColor[] vertices = new VertexPositionColor[3];
vertices[0].Position = new Vector3(-0.5f, -0.5f, 0f);
vertices[0].Color = Color.Red;
vertices[1].Position = new Vector3(0, 0.5f, 0f);
vertices[1].Color = Color.Green;
vertices[2].Position = new Vector3(0.5f, -0.5f, 0f);
vertices[2].Color = Color.Yellow;
GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(PrimitiveType.TriangleList, vertices, 0, 1);

但是，一旦我运行它，应用程序就会关闭，并抛出 InvalidOperationException。这是WP7应用程序。我错过了什么吗？提前感谢您的帮助。

Answer 1

The documentation表示 DrawUserPrimitives 在以下情况下抛出 InvalidOperationException:

A valid vertex shader and pixel shader was not set before calling DrawUserPrimitives. Both a valid vertex shader and pixel shader (or valid effect) must be set on the device before any draw operations may be performed.

(它还说如果你的顶点无效它会抛出 - 但它们对我来说看起来没问题。)

您需要在图形设备上设置一个Effect。具体来说，您需要在调用 DrawUserPrimitives 之前调用 EffectPass.Apply。一个简单的开始方法是使用 BasicEffect。下面是一些代码，适合放在 Draw 方法中，以说明这一点:

// These three lines are required if you use SpriteBatch, to reset the states that it sets
GraphicsDevice.BlendState = BlendState.Opaque;
GraphicsDevice.DepthStencilState = DepthStencilState.Default;
GraphicsDevice.SamplerStates[0] = SamplerState.LinearWrap;

// Transform your model to place it somewhere in the world
basicEffect.World = Matrix.CreateRotationZ(MathHelper.PiOver4) * Matrix.CreateTranslation(0.5f, 0, 0); // for sake of example
//basicEffect.World = Matrix.Identity; // Use this to leave your model at the origin
// Transform the entire world around (effectively: place the camera)
basicEffect.View = Matrix.CreateLookAt(new Vector3(0, 0, -3), Vector3.Zero, Vector3.Up);
// Specify how 3D points are projected/transformed onto the 2D screen
basicEffect.Projection = Matrix.CreatePerspectiveFieldOfView(MathHelper.ToRadians(45),
        (float)GraphicsDevice.Viewport.Width / (float)GraphicsDevice.Viewport.Height, 1.0f, 100.0f);

// Tell BasicEffect to make use of your vertex colors
basicEffect.VertexColorEnabled = true;
// I'm setting this so that *both* sides of your triangle are drawn
// (so it won't be back-face culled if you move it, or the camera around behind it)
GraphicsDevice.RasterizerState = RasterizerState.CullNone;

// Render with a BasicEffect that was created in LoadContent
// (BasicEffect only has one pass - but effects in general can have many rendering passes)
foreach(EffectPass pass in basicEffect.CurrentTechnique.Passes)
{
    // This is the all-important line that sets the effect, and all of its settings, on the graphics device
    pass.Apply();

    // Here's your code:
    VertexPositionColor[] vertices = new VertexPositionColor[3];
    vertices[0].Position = new Vector3(-0.5f, -0.5f, 0f);
    vertices[0].Color = Color.Red;
    vertices[1].Position = new Vector3(0, 0.5f, 0f);
    vertices[1].Color = Color.Green;
    vertices[2].Position = new Vector3(0.5f, -0.5f, 0f);
    vertices[2].Color = Color.Yellow;
    GraphicsDevice.DrawUserPrimitives<VertexPositionColor>(PrimitiveType.TriangleList, vertices, 0, 1);
}