Code examples of System.Linq.Expressions namespace class usage with explanations.

Topics

• Methods

• Constructors (Coming Soon)

• Fields (Coming Soon)

• Properties

• Blocks (Coming Soon)

• Variables (Coming Soon)

• Constants (Coming Soon)

• Casts (Coming Soon)

• Loops (Coming Soon)

• Conditions (Coming Soon)

Complete unit tests with compilable project can also be found here.

Sample Code Example for Calling Instance Method

This example shows the general flow of creating the lambda delegate for executing an instance method.

Fiddle: (https://dotnetfiddle.net/hFssOH)

Code:

DateTime testDate = DateTime.Parse("1/2/2020 1:23:45.678");

// locate the method to be called
MethodInfo toString = testDate
    .GetType()
    .GetMethod(
        "ToString",
        new Type[] { typeof(string) });

// ParameterExpression is used to map a value from the caller
// to the lambda body
ParameterExpression date = Expression.Parameter(
    testDate.GetType());

// use Expression.Constant to convert a 'normal' value into an expression
ConstantExpression format = Expression.Constant(
    "hh:mm:ss");			

// define the body of the lambda

// we're going to just have a single method call within the body
MethodCallExpression body = Expression.Call(
    date,
    toString,
    new Expression[] { format });

// Func<in T, out TResult>

// first Type argument of Func definition will be Type of instance object
// second Type argument of Func definition will be Type of return

Expression<Func<DateTime, string>> lambda = Expression.Lambda<Func<DateTime, string>>(

    // this is going to be the function body/logic
    
    // the value returned from a function is the last expression in its body
    body,

    // this is the parameter list being passed in to the lambda body

    // have to use same ParameterExpression instances referenced in
    // expression body definition above to get them to map through Func call
    new ParameterExpression[] { date });

// create the Func from the lambda
Func<DateTime, string> func = lambda.Compile();

// can now use Func to execute instance method on object instance
// and get result
string formattedDate = func(testDate);

Console.WriteLine(formattedDate);

Result:

01:23:45