这是我的LINQ查询
var listLogOutItems =
(from A in data
orderby A.FirstName
select new {
Login = "Logout",
Name = A.FirstName + " " + A.SurName,
ID = A.Id,
LogoutDate = A.LogOutTime.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)[0]
}).Distinct();
如果A.LogOutTime为null，则返回＆＃34; Unknown＆＃34;。怎么办呢？
答案 0 :(得分：3)
最简单的方法可能是使用空合并运算符??。
LogoutDate = (A.LogOutTime ?? "Unknown").Split(...)[0]
如果A.LogOutTime为null，则会使用"Unknown"代替Split。这样效率稍低，因为它会在我们知道不需要拆分的字符串上调用LogoutDate = A.LogOutTime?.Split(...)[0] ?? "Unknown"
...在C＃6中，您可以使用空条件运算符代替：
A.LogOutTime此处，如果null为Split，则null不会被评估，并且该部分表达式的结果将为char[] - 那么最后使用了null coalescing运算符。
顺便说一句，不是每次都创建一个新的private static readonly ArrayWithSpace = new[] { ' ' };
，你可能只有一个静态字段：
Split然后在每次调用var gl = null,
canvas = null,
glProgram = null,
fragmentShader = null,
vertexShader = null;
var coordinateArray = [ ],
triangleVerticeColors = [ ],
verticesArray = [ ],
verticesIndexArray = [ ];
var vertexPositionAttribute = null,
trianglesVerticeBuffer = null,
vertexColorAttribute = null,
trianglesColorBuffer = null,
triangleVerticesIndexBuffer = null;
var P = mat4.create(),
V = mat4.create();
M = mat4.create();
function initWebGL() {
canvas = document.getElementById("my-canvas");
try {
gl = canvas.getContext("webgl") ||
canvas.getContext("experimental-webgl");
} catch (e) {
}
if (gl) {
setupWebGL();
initShaders();
setupBuffers();
getMatrixUniforms();
setMatrixUniforms();
animationLoop();
//drawScene();
} else {
alert("Error: Your browser does not appear to" + "support WebGL.");
}
}
function animationLoop() {
var R = mat4.create();
var angle = 0;
var i = 0;
var loop = function() {
angle = performance.now() / 1000 / 6 * 2 * Math.PI;
i++;
mat4.rotate(M, R, angle, [ 0, 1, 0 ]);
gl.uniformMatrix4fv(glProgram.mvMatrixUniform, false, M);
gl.clearColor(0.1, 0.5, 0.1, 1.0);
gl.clear(gl.DEPTH_BUFFER_BIT | gl.COLOR_BUFFER_BIT);
drawScene();
requestAnimationFrame(loop);
};
requestAnimationFrame(loop);
}
function setupWebGL() {
gl.enable(gl.DEPTH_TEST);
gl.clearColor(0.1, 0.5, 0.1, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
console.log(P);
console.log(V);
console.log(M);
mat4.lookAt(V, [ 3, -1, -5 ], [ 0, 0, 0 ], [ 0, 1, 0 ]);
mat4.perspective(P, glMatrix.toRadian(45), canvas.width / canvas.height, 0.1, 1000.0);
}
function initShaders() {
var fs_source = document.getElementById('shader-fs').innerHTML,
vs_source = document.getElementById('shader-vs').innerHTML;
vertexShader = makeShader(vs_source, gl.VERTEX_SHADER);
fragmentShader = makeShader(fs_source, gl.FRAGMENT_SHADER);
glProgram = gl.createProgram();
gl.attachShader(glProgram, vertexShader);
gl.attachShader(glProgram, fragmentShader);
gl.linkProgram(glProgram);
if (!gl.getProgramParameter(glProgram, gl.LINK_STATUS)) {
alert("Unable to initialize the shader program.");
}
gl.useProgram(glProgram);
}
function makeShader(src, type) {
var shader = gl.createShader(type);
gl.shaderSource(shader, src);
gl.compileShader(shader);
if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
alert("Error compiling shader: " + gl.getShaderInfoLog(shader));
}
return shader;
}
function setupBuffers() {
// n-sides polygon
var n = 6;
var radius = 1;
var angle = (Math.PI * 2) / n;
var xCoordinate = 0;
var yCoordinate = 0;
for (var i = 0; i < n; i++) {
var a = angle * i;
var xNewCoordinate = xCoordinate + radius * Math.cos(a);
var yNewCoordinate = yCoordinate + radius * Math.sin(a);
var zNewCoordinate = 0;
coordinateArray.push(xNewCoordinate);
coordinateArray.push(yNewCoordinate);
coordinateArray.push(zNewCoordinate);
}
verticesArray = [
//Bottom Face
0.0, 0.0, 0.0,
0.0, 0.0, -1.0,
1.0, 0.0, -1.0,
0.0, 0.0, 0.0,
1.0, 0.0, -1.0,
1.0, 0.0, 0.0,
//Front Face
0.0, 0.0, 0.0,
1.0, 0.0, 0.0,
0.5, 1.0, -0.5,
//Right Face
1.0, 0.0, 0.0,
1.0, 0.0, -1.0,
0.5, 1.0, -0.5,
//Back Face
1.0, 0.0, -1.0,
0.0, 0.0, -1.0,
0.5, 1.0, -0.5,
//Left Face
0.0, 0.0, -1.0,
0.0, 0.0, 0.0,
0.5, 1.0, -0.5,
];
trianglesVerticeBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, trianglesVerticeBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(verticesArray), gl.STATIC_DRAW);
verticesIndexArray = [
3, 2, 1,
3, 1, 0,
3, 0, 4,
0, 1, 4,
1, 2, 4,
2, 3, 4,
];
triangleVerticesIndexBuffer = gl.createBuffer();
triangleVerticesIndexBuffer.number_vertext_points = verticesIndexArray.length;
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, triangleVerticesIndexBuffer);
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(verticesIndexArray), gl.STATIC_DRAW);
triangleVerticeColors = [
1.0, 0.0, 0.0,
1.0, 0.0, 0.0,
1.0, 0.0, 0.0,
0.0, 1.0, 0.0,
0.0, 1.0, 0.0,
0.0, 1.0, 0.0,
1.0, 0.0, 1.0,
1.0, 0.0, 1.0,
1.0, 0.0, 1.0,
0.5, 0.0, 0.0,
0.5, 0.0, 0.0,
0.5, 0.0, 0.0,
0.0, 5.0, 0.0,
0.0, 5.0, 0.0,
0.0, 5.0, 0.0,
1.0, 1.0, 0.0,
1.0, 1.0, 0.0,
1.0, 1.0, 0.0,
];
trianglesColorBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, trianglesColorBuffer);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(triangleVerticeColors), gl.STATIC_DRAW);
}
function getMatrixUniforms() {
glProgram.mvMatrixUniform = gl.getUniformLocation(glProgram, "uMVMatrix");
glProgram.pMatrixUniform = gl.getUniformLocation(glProgram, "uPMatrix");
glProgram.vMatrixUniform = gl.getUniformLocation(glProgram, "uVMatrix");
}
function setMatrixUniforms() {
gl.uniformMatrix4fv(glProgram.mvMatrixUniform, false, M);
gl.uniformMatrix4fv(glProgram.pMatrixUniform, false, P);
gl.uniformMatrix4fv(glProgram.vMatrixUniform, false, V);
}
function drawScene() {
vertexPositionAttribute = gl.getAttribLocation(glProgram, "aVertexPosition");
gl.enableVertexAttribArray(vertexPositionAttribute);
gl.bindBuffer(gl.ARRAY_BUFFER, trianglesVerticeBuffer);
gl.vertexAttribPointer(vertexPositionAttribute, 3, gl.FLOAT, false, 0, 0);
vertexColorAttribute = gl.getAttribLocation(glProgram, "aVertexColor");
gl.enableVertexAttribArray(vertexColorAttribute);
gl.bindBuffer(gl.ARRAY_BUFFER, trianglesColorBuffer);
gl.vertexAttribPointer(vertexColorAttribute, 3, gl.FLOAT, false, 0, 0);
gl.drawElements(gl.TRIANGLE_STRIP, triangleVerticesIndexBuffer.number_vertext_points, gl.UNSIGNED_SHORT, 0);
}
initWebGL();时重复使用它。