Resolve errors and warnings generated from expressions prohibited in expression trees

This article covers the following compiler errors:

• CS0765 - Partial methods with only a defining declaration or removed conditional methods cannot be used in expression trees.
• CS0831 - An expression tree may not contain a base access.
• CS0832 - An expression tree may not contain an assignment operator
• CS0834 - A lambda expression with a statement body cannot be converted to an expression tree.
• CS0835 - Cannot convert lambda to an expression tree whose type argument 'type' is not a delegate type.
• CS0838 - An expression tree may not contain a multidimensional array initializer.
• CS0845 - An expression tree lambda may not contain a coalescing operator with a null or default literal left-hand side.
• CS0853 - An expression tree may not contain a named argument specification.
• CS0854 - An expression tree may not contain a call or invocation that uses optional arguments.
• CS0855 - An expression tree may not contain an indexed property.
• CS1944 - An expression tree may not contain an unsafe pointer operation.
• CS1945 - An expression tree may not contain an anonymous method expression.
• CS1946 - An anonymous method expression cannot be converted to an expression tree.
• CS1951 - An expression tree lambda may not contain a ref, in or out parameter.
• CS1952 - An expression tree lambda may not contain a method with variable arguments.
• CS1963 - An expression tree may not contain a dynamic operation.
• CS1989 - Async lambda expressions cannot be converted to expression trees.
• CS2037 - An expression tree lambda may not contain a COM call with ref omitted on arguments.
• CS7053 - An expression tree may not contain "feature".
• CS8072 - An expression tree lambda may not contain a null propagating operator.
• CS8074 - An expression tree lambda may not contain a dictionary initializer.
• CS8075 - An extension Add method is not supported for a collection initializer in an expression lambda.
• CS8110 - An expression tree may not contain a reference to a local function.
• CS8122 - An expression tree may not contain an 'is' pattern-matching operator.
• CS8143 - An expression tree may not contain a tuple literal.
• CS8144 - An expression tree may not contain a tuple conversion.
• CS8153 - An expression tree lambda may not contain a call to a method, property, or indexer that returns by reference.
• CS8155 - Lambda expressions that return by reference cannot be converted to expression trees.
• CS8188 - An expression tree may not contain a throw-expression.
• CS8198 - An expression tree may not contain an out argument variable declaration.
• CS8207 - An expression tree may not contain a discard.
• CS8382 - An expression tree may not contain a tuple == or != operator.
• CS8514 - An expression tree may not contain a switch expression.
• CS8640 - Expression tree cannot contain value of ref struct or restricted type.
• CS8642 - An expression tree may not contain a null coalescing assignment.
• CS8790 - An expression tree may not contain a pattern System.Index or System.Range indexer access.
• CS8791 - An expression tree may not contain a from-end index ('^') expression.
• CS8792 - An expression tree may not contain a range ('..') expression.
• CS8810 - '&' on method groups cannot be used in expression trees.
• CS8849 - An expression tree may not contain a with-expression.
• CS8927 - An expression tree may not contain an access of static virtual or abstract interface member.
• CS8952 - An expression tree may not contain an interpolated string handler conversion.
• CS8972 - A lambda expression with attributes cannot be converted to an expression tree.
• CS9170 - An expression tree may not contain an inline array access or conversion.
• CS9175 - An expression tree may not contain a collection expression.
• CS9226 - An expression tree may not contain an expanded form of non-array params collection parameter.

Expression tree restrictions

All of the errors in the preceding list indicate you've used a C# expression type that isn't allowed in an expression tree. In most cases, the prohibited expressions represent syntax introduced after C# 3.0. These expressions are prohibited because allowing them would create a breaking change in all libraries that parse expression trees. All libraries would need to be enhanced to parse new C# expressions if newer constructs were allowed.

The following expressions are prohibited:

• Invocations of partial methods that don't have an implementing declaration.
• Invocations of conditional methods that have been removed.
• Invocations of local functions.
• async lambda expressions aren't allowed.
• Using base access to directly call a virtual method declared in a base class.
• assignment operations.
• statement lambdas aren't allowed.
• multi-dimensional array initializers. Instead, you must create and initialize a multi-dimensional array outside of the expression tree.
• dynamic operations aren't allowed.
• pattern matching expressions aren't allowed.
• Tuple literals and many tuple operations, such as equality comparisons aren't allowed.
• throw expressions aren't allowed.
• discard (_) declarations.
• The index and range operators aren't allowed.
• Non-destructive mutation using with expressions aren't allowed.
• You can't declare or access inline arrays.
• You can't include collection expressions.
• The null propagating and null coalescing operators aren't allowed.
• ref struct types, such as System.Span<T> and System.ReadOnlySpan<T> aren't allowed.
• in, out, and ref parameters, including out variable declarations, aren't allowed.
• ref returns aren't allowed.
• Calls to methods that return by ref aren't allowed.
• static abstract interface members can't be accessed.
• Inline arrays.
• The params modifier is allowed only on single-dimensional arrays. Other collection types aren't allowed.

Other restrictions are:

• Attributes can't be applied to the lambda expression, its parameters or return.
• The lambda expression must be convertible to a type derived from System.Linq.Expressions.Expression whose type parameter is a delegate type.
• named and optional parameters are restricted. The expression can't call a method specifying named arguments, and it can't use the default value of an optional parameter.
• Dictionary initializers aren't allowed. Neither are extension Add methods.
• The target expression must be a lambda expression. Constants and variables aren't allowed, but a lambda expression that returns a constant or variable is.
• Unsafe pointer operations aren't allowed.
• COM calls must include ref on arguments; it can't be implied.
• The unsupported __arglist keyword isn't allowed.